Update AssertJ to 3
Since AssertJ 3 relies on Java 8, we can now update on master. It seems that there are very few breaking changes so hopefully this should not be a big hassle.

build.gradle
build.gradle
diff --git a/build.gradle b/build.gradle
index 33f03bff72..946fd0d943 100644
--- a/build.gradle
+++ b/build.gradle
@@ -67,7 +67,7 @@ dependencies {
 
     testCompile 'org.ow2.asm:asm:5.1'
 
-    testCompile 'org.assertj:assertj-core:1.7.1'
+    testCompile 'org.assertj:assertj-core:3.5.2'
 
     testRuntime configurations.provided
 
