Added Yosemite (10.10) to list of supported OS releases

Hey Boxenites,

I added Yosemite (10.10) to the list of support versions in the OS preflight check. I had a poke around for any related tests, but couldn't find anything. Let me know if there's anything I missed for this PR.

Also, thanks for all your hard work on Boxen! I love it :cake: