Initial Yosemite Support

Closes Homebrew/homebrew#31435.

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

Library/Homebrew/os/mac.rb

@@ -206,6 +206,7 @@ module OS
       "5.0.2" => { :clang => "5.0", :clang_build => 500 },
       "5.1"   => { :clang => "5.1", :clang_build => 503 },
       "5.1.1" => { :clang => "5.1", :clang_build => 503 },
+      "6.0"   => { :clang => "6.0", :clang_build => 600 },
     }
 
     def compilers_standard?

Library/Homebrew/os/mac/xcode.rb

@@ -14,10 +14,11 @@ module OS
         when "10.7"         then "4.6.3"
         when "10.8"         then "5.1.1"
         when "10.9"         then "5.1.1"
+        when "10.10"        then "6.0"
         else
           # Default to newest known version of Xcode for unreleased OSX versions.
           if MacOS.version > "10.9"
-            "5.1.1"
+            "6.0"
           else
             raise "Mac OS X '#{MacOS.version}' is invalid"
           end
@@ -115,6 +116,7 @@ module OS
           when 42      then "4.6"
           when 50      then "5.0"
           when 51      then "5.1"
+          when 60      then "6.0"
           else "5.1"
           end
         end