Add caching to travis

https://docs.travis-ci.com/user/languages/android#Caching

.travis.yml

@@ -17,3 +17,10 @@ before_script:
 #Build, and run tests
 script: "./gradlew clean assembleDebug testDebugUnitTest"
 sudo: false
+
+before_cache:
+  - rm -f $HOME/.gradle/caches/modules-2/modules-2.lock
+cache:
+  directories:
+    - $HOME/.gradle/caches/
+    - $HOME/.gradle/wrapper/