Add JS runtimes for asset pipeline build

In production, we do "bundle exec rake assets:precompile" to build assets during the deploy process.  This requires a JS runtime; other teams here are using execjs + therubyracer so this pull request does the same.

IMPORTANT: could conceivably break Heroku -- no idea how things work there.  Please test before merging if you've got a place where you can do so, or feel free to wait till we're close to production-ready before merging this.  (I'm testing w/Vagrant and my own fork of zengrid to get the Chef stuff confirmed working.)