Fix typo Os -> OS X, Drop the 'Mac'

Apple officially renamed Mac OS X to OS X.

http://www.theverge.com/2012/2/16/2802281/apple-officially-renames-mac-os-x-to-os-x-drops-the-mac
Merge pull request #200 from graingert/patch-2

Fix typo Os -> OS X, Drop the 'Mac'