Remove Gemfile

The only things installed by the Gemfile are Vagrant 1.0.5 and Sahara
(which depends on Vagrant 1.0.5).

We don't use the Vagrant gem any more, so running bundle install with this
Gemfile installed a version of Vagrant that was too old to be any use.
Merge pull request #10 from alexmuller/remove-gemfile

Remove Gemfile