WIP: load gherkin asynchronously to fix JS errors on load with production resources.

* Copy all bower components to prod so gherkin is available in prod mode.
* load gherkin asynchronously

@zaach - this is to load gherkin asynchronously to avoid the JS errors with production resources. There are still problems with this, like the bundle is not minified on build, but it does avoid the JS error on startup.

I'm not sure whether this is worth merging, it is an idea.

It's worth viewing the diff with ?w=1 - https://github.com/mozilla/fxa-content-server/pull/165/files?w=1