Support Scala 2.11 in the build definition file.

Fix pattern matches to work with Scala 2.10 and 2.11 by just using the
wildcard case.
Update to sbt 0.13.1.

Also, update to latest versions of sbt plugins used in spray-json build.
Merge pull request #97 from gkossakowski/scala-2.11

Support for Scala 2.11