Factor out a `vagrant` method
Replace filesystem test with vagrant box list

Vagrant 1.5 added box version support, which changes the directory
structure that we were expecting. As slow as it is, we should use
vagrant's `box list` command to test which boxes are installed, rather
than depending on knowledge of its private directory structure.
upgrade dependencies, fix spec test
Merge pull request #46 from JHaals/master

upgrade dependencies, fix spec test
Merge pull request #45 from boxen/fix-box-detection

Fix box detection
merge conflict