Fixes #35 - Bump to sbt 0.12.4 for old versions.

Here we bump to sbt 0.12.4 when forking sbt so that the apply -cp
hooks are available.  For all projects used in activator, this is a
safe operation, and should remain so.
Merge pull request #36 from sbt/wip-jsuereth-bump-to-0.12.4-when-needed

Fixes #35 - Bump to sbt 0.12.4 for old versions.