Use sbt 0.13 for building zinc.

Let's get rid of override of sbt version when building zinc. Zinc has
already migrated to sbt 0.13 and it doesn't have to be treated specially.
Remove sbt-version-overrides in each project.

We can override sbt version at the build level which propagates to
all projects defined in given build. It cuts down on boilerplate in the
config.
Merge pull request #62 from gkossakowski/zinc-breakage

Fix Zinc breakage