``Table.sort`` failing on MacOS X

Thanks to the doctests that @healther added, Jenkins found that ``Table.sort`` also does not work correctly on MacOS X with Numpy < 1.7:

https://jenkins.shiningpanda-ci.com/astropy/view/MacOS%20X/job/astropy-master-osx-10.8-multiconfig/

@taldcroft - is there an easy way to fix this one?