Add caching to Travis

.travis.yml

@@ -8,3 +8,10 @@ jdk:
   - oraclejdk8
   - oraclejdk7
   - openjdk6
+# Caching for Gradle files, prevents hitting Maven too much.
+before_cache:
+  - rm -f $HOME/.gradle/caches/modules-2/modules-2.lock
+cache:
+  directories:
+    - $HOME/.gradle/caches/
+    - $HOME/.gradle/wrapper/