Revert "Make sure 10.9 users upgrade XQuartz"

This causes CI builds on 10.9 to fail; will repush once it is updated.

This reverts commit 70c5ce09d995426aefdbfd4aba0b06efe100a433.

Library/Homebrew/os/mac/xquartz.rb

@@ -39,14 +39,8 @@ module OS
         end
       end
 
-      # http://xquartz.macosforge.org/trac/wiki
-      # http://xquartz.macosforge.org/trac/wiki/Releases
       def latest_version
-        case MacOS.version
-        when "10.5" then "2.6.3"
-        when "10.9" then "2.7.5_rc4"
-        else "2.7.4"
-        end
+        "2.7.4"
       end
 
       def bundle_path