[beta] Do not manually install git && bundler.

* Remove manual git installation.  https://github.com/travis-ci/travis-ci/issues/1710
  has been closed, and Travis VM's should now have git 1.8.5.3.

* Bundler 1.5+ is installed by default on Travis now, no need to install
  manually.
Merge pull request #4333 from rjackson/dont-install-git-manually

[beta] Do not manually install git && bundler.