Remove runtime maven dep

Limit Maven dependencies to MavenDependencyResolver rather than letting the Maven implementation leak through the API.

This means for configurations (like here at Google) where we do not use Maven for providing dependencies, it is no longer necessary to provide a runtime dependency on Maven (~200k) which we'd otherwise be shipping round the network for each test invocation.
This is a nit, but can use remove the file header? I assume your contribution can be licensed as MIT (the same as the rest of Robolectric)?