Updated specs2 section in README.

Updated README doc. The unfiltered module supporting specs2 is now called `specs2` instead of `specs`.

Also, there is a similarly outdated line in the docs at http://unfiltered.databinder.net/Combined+Pages.html#Project+Setup. Is there somewhere I can go to submit a PR to update that?

Specifically, those docs suggest that `unfiltered-spec_2.11.0` is the correct artifact name. But really it is `unfiltered-specs2_2.11.0 `.

Thanks.