Cross-compile to scala 2.11.0-RC1.

First release candidate of scala 2.11 is [out](https://groups.google.com/forum/#!topic/scala-language/KomfLntCbcU). This adds it to the cross-compilation targets.

**P.S.** I would really appreciate a release of this library for 2.11: my company is using twitter's stack, which all depends on scalatest :cat: