Update build files to work with Android Studio 1.0

This PR changes the build files in that way that everyone who checks out the project can build it with the current tools:

- include a debug keystore and link it via signingConfig
- update to android-gradle-plugin 1.0.0 final
- update wrapper to Gradle 2.2.1, as required by android-gradle-plugin
- update all used libraries to their current versions (21.x, current play services)
- update compileSDK to 21
- update travis config to use current tools, Google m2 repo and SDK