Cache the gradle wrapper in travis builds.

From Gradle's [Enable caching of downloaded artifacts](https://guides.gradle.org/executing-gradle-builds-on-travisci/#enable_caching_of_downloaded_artifacts).

Author: Capstan

.travis.yml

@@ -6,3 +6,12 @@ install: {}
 script:
   - ./gradlew assemble check
 
+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/
+