Do not use Travis variables in GitSupport.

We were using the Travis variables in two areas:

* commit_range - The range actually provided by Travis (from GitHub's
  webhook to them) is not a valid commit range (can not do `git log
  <commit range>`)
* current_branch - We have added the logic to determine the current
  branch directly (to support multi-branch testing).

This ultimately removes some complexity, and resolves a number of build
failures that I have been dealing with when integrating the latest
ember-dev into Ember itself.