Add yosemite to mac versions

Closes Homebrew/homebrew#29823.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/os/mac/version.rb

@@ -4,6 +4,7 @@ module OS
   module Mac
     class Version < ::Version
       SYMBOLS = {
+        :yosemite      => '10.10',
         :mavericks     => '10.9',
         :mountain_lion => '10.8',
         :lion          => '10.7',