Restored write_if_different per https://github.com/astropy/astropy-helpers/pull/110#issuecomment-66697634 (I thought it was unused since it is not used within astropy_helpers, but it is being used externally.  However, tehre were actually two copies of this function in astropy_helpers, somehow.  One in setup_helpers and one in utils.  I have added it back to utils but also included it in setup_helpers via import.