Gradle caching

Change-Id: I883fa6da8eceb613f734306d6355e23c4aead451

Author: samtstern

.travis.yml

@@ -17,6 +17,14 @@ android:
     - 'android-sdk-license-.+'
     - 'google-gdk-license-.+'
 
+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/
+
 env:
   - SAMPLE=admob 
   - SAMPLE=analytics