Account for Play Services dependency on the support library.

Fixes #31 by making sure that the support library repository is present when
trying to resolve dependencies on Play Services, without which an exception was
thrown causing the plugin to think that Play Services was always outdated. Also
updates tests to reflect this dependency and fail without the fix.
Merge pull request #39 from alexsydell/fix-play-services-support-dependency

Account for Play Services dependency on the support library.