Update requested sbt version to make it able to fetch scala-xml in the right place.

Sbinary used sbt 0.12.4 which has the wrong default for
scalaBinaryVersion for Scala milestones. This requires sbt 0.13.0 for
the build, making it fetch scala-xml at a URI that takes the full
*milestone* version string into account.
Merge pull request #5 from huitseeker/update-build

Update requested sbt version to make it able to fetch scala-xml in the right place.