update sbt 0.13.2 and sbt plugins versions

sbt 0.13.2 has been released.
Let's bump up version!
But I didn't update `sbt-unidoc` plugin.
`sbt-unidoc` plugin 0.3.0 contains breaking changes as follows:

> String-based __excludedProjects__ is now replaced with __unidocProjectFilter__. 

see more detail: http://notes.implicit.ly/post/63162383096/sbt-unidoc-0-3-0

Using `sbt-unidoc` 0.3.0 for scalaz, it need more work....