diff --git a/.travis.yml b/.travis.yml
index cffa9ba..368f17e 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,12 +1,13 @@
 language: python
-python: 2.7
-env:
-  - TOX_ENV=py26
-  - TOX_ENV=py27
-  - TOX_ENV=py33
-  - TOX_ENV=py34
-install: pip install tox coveralls
-before_script: tox -e flake8
-script: tox -e ${TOX_ENV}
+python:
+  - 2.6
+  - 2.7
+  - 3.3
+  - 3.4
+  - 3.6
+before_install: pip install --upgrade setuptools
+install: pip install tox tox-travis coveralls
+before_script: if [[ $TRAVIS_PYTHON_VERSION == '2.7' ]]; then tox -e flake8; fi
+script: tox -r
 after_success: coveralls
 sudo: false
diff --git a/pyhocon/config_tree.py b/pyhocon/config_tree.py
index ca015ff..6993aef 100644
--- a/pyhocon/config_tree.py
+++ b/pyhocon/config_tree.py
@@ -220,6 +220,8 @@ class ConfigTree(OrderedDict):
         """
         value = self.get(key, default)
         if value == default:
+            if key in self:
+                del self[key]
             return default
 
         lst = ConfigTree.parse_key(key)
diff --git a/pyhocon/tool.py b/pyhocon/tool.py
index 8620fc2..241fd35 100644
--- a/pyhocon/tool.py
+++ b/pyhocon/tool.py
@@ -3,6 +3,8 @@ import logging
 import sys
 from pyhocon import ConfigFactory
 from pyhocon.config_tree import ConfigTree
+from pyhocon.config_tree import NoneValue
+
 
 try:
     basestring
@@ -52,7 +54,7 @@ class HOCONConverter(object):
                 lines += '\n{indent}]'.format(indent=''.rjust(level * indent, ' '))
         elif isinstance(config, basestring):
             lines = '"{value}"'.format(value=config.replace('\n', '\\n').replace('"', '\\"'))
-        elif config is None:
+        elif config is None or isinstance(config, NoneValue):
             lines = 'null'
         elif config is True:
             lines = 'true'
@@ -103,7 +105,7 @@ class HOCONConverter(object):
                 lines = '"""{value}"""'.format(value=config)  # multilines
             else:
                 lines = '"{value}"'.format(value=config.replace('\n', '\\n').replace('"', '\\"'))
-        elif config is None:
+        elif config is None or isinstance(config, NoneValue):
             lines = 'null'
         elif config is True:
             lines = 'true'
@@ -150,6 +152,8 @@ class HOCONConverter(object):
                 lines = config
             else:
                 lines = '|\n' + '\n'.join([line.rjust(level * indent, ' ') for line in lines])
+        elif config is None or isinstance(config, NoneValue):
+            lines = 'null'
         elif config is True:
             lines = 'true'
         elif config is False:
@@ -185,6 +189,8 @@ class HOCONConverter(object):
             lines.append('.'.join(stripped_key_stack) + ' = true')
         elif config is False:
             lines.append('.'.join(stripped_key_stack) + ' = false')
+        elif config is None or isinstance(config, NoneValue):
+            pass
         else:
             lines.append('.'.join(stripped_key_stack) + ' = ' + str(config))
         return '\n'.join([line for line in lines if len(line) > 0])
diff --git a/tox.ini b/tox.ini
index f9a71ce..5a4f7da 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,5 +1,5 @@
 [tox]
-envlist = flake8, py26, py27, py33, py34
+envlist = flake8, py26, py27, py33, py34, py36
 
 [testenv]
 passenv = TRAVIS TRAVIS_JOB_ID TRAVIS_BRANCH

diff --git a/tests/test_config_tree.py b/tests/test_config_tree.py
index f9016d1..97a8e17 100644
--- a/tests/test_config_tree.py
+++ b/tests/test_config_tree.py
@@ -237,6 +237,11 @@ class TestConfigParser(object):
         assert config_tree.pop("config-new", {'b': 1}) == {'b': 1}
         assert config_tree == ConfigTree()
 
+        config_tree = ConfigTree()
+        config_tree.put('key', 'value')
+        assert config_tree.pop('key', 'value') == 'value'
+        assert 'key' not in config_tree
+
         config_tree = ConfigTree()
         config_tree.put('a.b.c.one', 1)
         config_tree.put('a.b.c.two', 2)
diff --git a/tests/test_tool.py b/tests/test_tool.py
index 937f9e7..9610d77 100644
--- a/tests/test_tool.py
+++ b/tests/test_tool.py
@@ -81,7 +81,7 @@ class TestHOCONConverter(object):
             f1: true
             f2: false
             g: []
-            h: None
+            h: null
             i:
             a.b: 2
         """

HOCONConverter.to_json can't convert `null` value
After convert into JSON with `HOCONConverter.to_json`, the value should be kept `null`, but it is converted to `NonValue`.

```py
In [19]: hocon = "{foo = null}"
In [20]: conf = ConfigFactory.parse_string(hocon)
In [21]: conf
Out[21]: ConfigTree([('foo', <pyhocon.config_tree.NoneValue at 0x1a1074e4fd0>)])
In [22]: HOCONConverter.to_json(conf)
Out[22]: '{\n  "foo": <pyhocon.config_tree.NoneValue object at 0x000001A1074E4FD0>\n}'
```
tests/test_config_tree.py::TestConfigParser::test_configtree_pop tests/test_tool.py::TestHOCONConverter::test_to_json tests/test_tool.py::TestHOCONConverter::test_to_yaml tests/test_tool.py::TestHOCONConverter::test_to_properties tests/test_tool.py::TestHOCONConverter::test_to_hocon tests/test_tool.py::TestHOCONConverter::test_convert_from_file
tests/test_config_tree.py::TestConfigParser::test_config_tree_quoted_string tests/test_config_tree.py::TestConfigParser::test_config_list tests/test_config_tree.py::TestConfigParser::test_config_tree_number tests/test_config_tree.py::TestConfigParser::test_config_tree_iterator tests/test_config_tree.py::TestConfigParser::test_config_logging tests/test_config_tree.py::TestConfigParser::test_config_tree_null tests/test_config_tree.py::TestConfigParser::test_getters tests/test_config_tree.py::TestConfigParser::test_getters_with_default tests/test_config_tree.py::TestConfigParser::test_getter_type_conversion_string_to_bool tests/test_config_tree.py::TestConfigParser::test_getter_type_conversion_bool_to_string tests/test_config_tree.py::TestConfigParser::test_getter_type_conversion_number_to_string tests/test_config_tree.py::TestConfigParser::test_overrides_int_with_config_no_append tests/test_config_tree.py::TestConfigParser::test_overrides_int_with_config_append tests/test_config_tree.py::TestConfigParser::test_plain_ordered_dict tests/test_config_tree.py::TestConfigParser::test_contains tests/test_config_tree.py::TestConfigParser::test_contains_with_quoted_keys tests/test_config_tree.py::TestConfigParser::test_keyerror_raised tests/test_tool.py::TestHOCONConverter::test_invalid_format