Prime the Ivy cache

The PR seeks to reduce the build time and hopefully eliminate some build timeouts by priming the Ivy cache prior to compiling an app for the first time.  Result show a savings of about 5 minutes for a vanilla Play app. It uses the [sbt-pkg-builder](https://github.com/heroku/sbt-pkg-builder/) to create the pre-built ivy tarballs.

In addition to priming the cache, this PR also splits the `sbt` commands into an `sbt update` followed by an `sbt compile stage`. This allows it to cache the Ivy repo before the app compiling starts.  This is important for some users who may get a timeout. Now a second `git push` will succeed because all dependencies have been resolved.

Once this is merged it, I'll follow up with more tarball ivy packages for various Play and Scala versions.
lol sandlot! so i'd still have to parse the output of `sbt play-version` (could be play < 2.3.x) to see if it matches any of the "supported" versions (i.e. versions we've built a pre-packaged cache for). Also, running `sbt` actually requires using sbt-launch.jar... so theoretically there's more chance of something going wrong. I'll play around see what I can do with it.