Upgrade sbt to 0.13.1.

It seems like everything works just fine so let's upgrade to latest
stable version.

This upgrades sbt version used for `sbt-republish` project itself and
the version this project republishes.
Bump republished version to 0.13.2-SNAPSHOT

The 0.13.1 has been already released so let's stick to latest SNAPSHOT.
Merge pull request #8 from gkossakowski/master

Upgrade to latest SNAPSHOT version of Sbt