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.
I'm not a Rubyist and this might be a personal preference but I'd prefer to use `if` instead of `unless` logic. It keeps things consistent within the code and I find it more readable since the test condition comes up front, not at the end. That said, if this construction is idiomatic Ruby, I have no argument then.