Upgraded to SBT 0.13

* Actually upgraded to 0.13.1-RC2 to address
  https://github.com/sbt/sbt/issues/937
* Also had to upgrade to specs 2.3.1 to work around issue with scalaz
  pulling in Scala 2.10.1 and SBT not honouring our scala version
* Upgraded scalariform, which meant some files change formatting
* SBT runs forked tests in a different directory, changed the tests that
  failed as a result to hopefully be more resilient to working directory
  changes
* Implements #1643
* Also fixed #1710 - caching is now used for scala/javadoc compilation
Merge pull request #1643 from jroper/1643-upgrade-013

Upgrade Play build to SBT 0.13