Fixed already defined warning
 * Changed how module shortcut is defined. The shortcut is now defined
   inside the class using it to avoid already defined warnings.

   Fixes [issue #24](https://github.com/maoueh/nugrant/issues/24).
 * Fixed auto export variables on `vagrant provision` feature. The
   initial release is not working correctly.
Merge branch 'fix-already-defined-warning'
Updated Gemfile vagrant dependency
 * Now always testing on master.
 * Updated .travis.yml by copying vagrant-berkshelf one (almost).