[WIP] Upgrade minimum Vagrant version from 1.1 to 1.5

@smashwilson / @mdarby
WIP but ready for review. I want to get your review and feedback on the changes so far, but check w/ me before merging so I can do one last review and cleanup.

Fixes #125.
Vagrant comes with an embedded Ruby. I believe it's been 2.0 since Vagrant 1.4.0.