Fix Zinc breakage

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.

The second commit includes some cleanups related to sbt-version-overrides. W 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.