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.
I actually deleted the `play-version` setting from master not long ago, so this won't work in Play 2.4.  Are you seeing any instances where a project doesn't have an `addSbtPlugin` for Play in `plugins.sbt`?

Note that running sbt to get the Play version will resolve/download a significant amount of the ivy cache, making this optimisation less effective.