Add support for google local repository

[pjakubczyk]

Error output:

• What went wrong:
  Could not resolve all dependencies for configuration ':robospock-sample:testCompile'.

  Could not find com.android.support:appcompat-v7:19.0.1.
  Required by:
  robospock-plugin:robospock-sample:unspecified

Workaround:
repositories {
mavenCentral()
maven { url "${System.env.ANDROID_HOME}/extras/android/m2repository" }
maven { url "${System.env.ANDROID_HOME}/extras/google/m2repository" }
}