Use _.bind in place of fn.bind

[ci skip]
Travis: Use Docker-based CI env

Also remove unnecessary path setting
Track event metadata in flush queue to allow individual events to be flushed
Rewrite flush queue updates

* style fixes
* change fn signature for #push to match [].push
* add test coverage for changes
Rewrite API docs for #getFlushQueue
Style fixes for fakeEvent
Style fixes for updates to Firebase methods