Enable caching on Travis

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

Author: valfirst

.travis.yml

@@ -41,3 +41,11 @@ matrix:
 # - adb devices
 # script:
 # - ./gradlew clean test --tests "io.appium.java_client.android.*" --exclude-task signArchives
+
+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/