Added a runtime check for the Android Gradle plugin version

From now on the plugin will fail if an unsupported Android Gradle plugin is detected. For the upcoming `0.14.0` release of the robolectric-gradle-plugin we support all `0.14.x` releases.

Refs #14 and #79 
After calling `./gradlew test` a new folder `src/test/fixtures/android_app/userHome` is created.