Build against astropy master

It looks like much of the code in `photutils`' `setup.py` script has been
refactored into `astropy.setup_helpers`. This commit fixes the following
error in `setup.py`:

    ERROR: TypeError: adjust_compiler() takes exactly 1 argument (0 given)
    [__main__]
    Traceback (most recent call last):
      File "setup.py", line 47, in <module>
        setup_helpers.adjust_compiler()