[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.
I replaced this with the built-in WaitForCommunicator middleware. I guess we lost the custom message (`vagrant_rackspace.waiting_for_communicator`) that showed the server's IP address....