Serve notifier.js locally as airbrake-notifier.js

We've run into the need to serve `notifier.js` locally due to periodic long responses from the hosted version of that js file. We've added `notifier.js` into `lib/assets` and switched `javascript_notifier.erb` to use this new local asset.

Obviously this places an added bit of work on the airbrake team to update their JS in this repo (or link git repo's appropriately), but I wanted to at least forward on this suggestion.