Switch to babel-schmooze-sprockets

[jcoyne]

https://github.com/fnando/babel-schmooze-sprockets

This would give us Babel 6 support. The current ruby-babel-transpiler doesn't support Babel 6 and the maintainer doesn't seem interested in it. babel/ruby-babel-transpiler#288

[tpendragon]

Oh, nice, it even has polyfill support. We had to pull that in before from rails-assets..