Merge pull request #550 from Mathias Düsterhöft

* gh-550:
  Configure Gradle caching for Travis CI builds

.travis.yml

@@ -4,5 +4,12 @@ env:
 language: java
 jdk:
   - oraclejdk8
+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/
 script:
   - ./gradlew build buildSamples --continue