Update README.md

Apple has renamed Mac OS X to OS X.
The official name for Yosemite is OS X Yosemite: https://www.apple.com/osx/

The more general OS X description is here: https://www.apple.com/osx/what-is/

The `OS X` name has been used since Mountain Lion (10.8): http://en.wikipedia.org/wiki/OS_X_Mountain_Lion#History (first paragraph)