Updates to GitGutter

* Change to use tags for releases
* Point a new *Edge* version to master
* Pointer a new *Release-Candidate* version to the release-candidate branch

This will allow users to easily install and test out new features before pushed onto the masses.

Wanted to make it much easier for uses to beta test new releases. If there is a better way in the Package Control Ecosystem to achieve this, please let me know.