sprockets-es6 dependency is experimental

[awead]

Descriptive summary

According to the sprockets-es6 github page:

This plugin is primarily experimental and will never reach a stable 1.0.
The purpose is to test out BabelJS features on Sprockets 3.x and include it by default in Sprockets 4.x.

I know there's probably nothing else out there at the moment because ECMA6 is pretty new, but I'm just registering the issue here so that we're aware we're depending on a self-described experimental gem.

[jcoyne]

And we will stop using it when Sprockets 4.0 is released (it's currently 4.0.0.beta2)