Ansible version check

Resubmitting #964 against the dev branch.

In #454 @smashwilson and @ycombinator discussed adding fail-fast logic to the Vagrantfile to check if the Ansible version is too old to work. If you still want it, this PR does exactly that.
As a Rubyist, using `if` vs. `unless` is a question of "what reads more naturally." As long as you never use `unless ... else ... end` you're in the clear :wink:

With that being said, personally I'd think this line's a bit long for a postfix `unless`. That's a nitpick though :stuck_out_tongue: