Remove JS notifier

This PR removes all the usage of the JavaScript notifier in the gem. 
After this is merged, I will update https://github.com/airbrake/airbrake/wiki/Javascript-support to point users to the official JS notifier https://github.com/airbrake/airbrake-js.

This will be part of a minor release.

This affects PRs https://github.com/airbrake/airbrake/pull/127, https://github.com/airbrake/airbrake/pull/229 and issues https://github.com/airbrake/airbrake/issues/66, https://github.com/airbrake/airbrake/issues/244