Fix comparison of Xcode version

The methods _XcodeVersion returns a tuple with two strings (Xcode
version and Xcode build), so we should check the version against
the first value of the tuple.

BUG=None
R=mark@chromium.com

git-svn-id: http://gyp.googlecode.com/svn/trunk@1848 78cadc50-ecff-11dd-a971-7dbc132099af