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.
Good point...  It would be nice if there was a dependency-less way to get build metadata.  Maybe there is.  Getting the list of deps would be a safe way to get this info.  But you are right that it's likely that 99% of folks probably do `addSbtPlugin` in a `project/plugins.sbt` file.  So might not be worth finding a different way right now.