Upgrade to sbt 0.13.2.

Sbt 0.13.2 includes name hashing which is being used (at the time this
commit is being created) by specs2 and akka. Let's upgrade to sbt 0.13.2
and unbreak community build at the same time.
Remove declaration of sbt 0.12.4.

No project in community-build is using sbt 0.12.4 anymore. Let's remove
the stale declaration of the sbt version override.
Merge pull request #33 from gkossakowski/sbt-0.13.2

Sbt 0.13.2