rails server error on ubuntu
<p>I am following ruby on rails getting started guide ( <a href="http://guides.rubyonrails.org/getting_started.html" rel="nofollow noreferrer">http://guides.rubyonrails.org/getting_started.html</a> )
I installed ruby and rails as it is suggested in the guide but when I try to execute:  </p>

<pre><code>bin/rails server
</code></pre>

<p>I see the following error in the terminal:</p>

<pre><code>Warning: Running `gem pristine --all` to regenerate your installed gemspecs (and deleting then reinstalling your bundle if you use bundle --path) will improve the startup performance of Spring.
/var/lib/gems/2.1.0/gems/bundler-1.11.2/lib/bundler/runtime.rb:80:in `rescue in block (2 levels) in require': There was an error while trying to load the gem 'uglifier'. (Bundler::GemRequireError)
from /var/lib/gems/2.1.0/gems/bundler-1.11.2/lib/bundler/runtime.rb:76:in `block (2 levels) in require'
from /var/lib/gems/2.1.0/gems/bundler-1.11.2/lib/bundler/runtime.rb:72:in `each'
from /var/lib/gems/2.1.0/gems/bundler-1.11.2/lib/bundler/runtime.rb:72:in `block in require'
from /var/lib/gems/2.1.0/gems/bundler-1.11.2/lib/bundler/runtime.rb:61:in `each'
from /var/lib/gems/2.1.0/gems/bundler-1.11.2/lib/bundler/runtime.rb:61:in `require'
from /var/lib/gems/2.1.0/gems/bundler-1.11.2/lib/bundler.rb:99:in `require'
from /home/soroosh/rails/blog/config/application.rb:7:in `&lt;top (required)&gt;'
from /var/lib/gems/2.1.0/gems/railties-4.2.5.1/lib/rails/commands/commands_tasks.rb:78:in `require'
from /var/lib/gems/2.1.0/gems/railties-4.2.5.1/lib/rails/commands/commands_tasks.rb:78:in `block in server'
from /var/lib/gems/2.1.0/gems/railties-4.2.5.1/lib/rails/commands/commands_tasks.rb:75:in `tap'
from /var/lib/gems/2.1.0/gems/railties-4.2.5.1/lib/rails/commands/commands_tasks.rb:75:in `server'
from /var/lib/gems/2.1.0/gems/railties-4.2.5.1/lib/rails/commands/commands_tasks.rb:39:in `run_command!'
from /var/lib/gems/2.1.0/gems/railties-4.2.5.1/lib/rails/commands.rb:17:in `&lt;top (required)&gt;'
from /home/soroosh/rails/blog/bin/rails:9:in `require'
from /home/soroosh/rails/blog/bin/rails:9:in `&lt;top (required)&gt;'
from /var/lib/gems/2.1.0/gems/spring-1.6.3/lib/spring/client/rails.rb:28:in `load'
from /var/lib/gems/2.1.0/gems/spring-1.6.3/lib/spring/client/rails.rb:28:in `call'
from /var/lib/gems/2.1.0/gems/spring-1.6.3/lib/spring/client/command.rb:7:in `call'
from /var/lib/gems/2.1.0/gems/spring-1.6.3/lib/spring/client.rb:28:in `run'
from /var/lib/gems/2.1.0/gems/spring-1.6.3/bin/spring:49:in `&lt;top (required)&gt;'
from /var/lib/gems/2.1.0/gems/spring-1.6.3/lib/spring/binstub.rb:11:in `load'
from /var/lib/gems/2.1.0/gems/spring-1.6.3/lib/spring/binstub.rb:11:in `&lt;top (required)&gt;'
from /usr/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
from /usr/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
from /home/soroosh/rails/blog/bin/spring:13:in `&lt;top (required)&gt;'
from bin/rails:3:in `load'
from bin/rails:3:in `&lt;main&gt;'
</code></pre>

<p>I am using ubuntu 15.10 , ruby 2.1 and rails4.2.5.1</p>