Added gradle caching, should speed up build

.travis.yml

@@ -1,9 +1,14 @@
 language: java
 jdk: oraclejdk8
-
 install: "./gradlew setupCIWorkspace"
 script: "./gradlew build"
 after_success: "./gradlew bintrayUpload"
-
+before_cache:
+  - rm -f  $HOME/.gradle/caches/modules-2/modules-2.lock
+  - rm -fr $HOME/.gradle/caches/*/plugin-resolution/
+cache:
+  directories:
+    - $HOME/.gradle/caches/
+    - $HOME/.gradle/wrapper/
 notifications:
   email: false