pyregion import in tests

[keflavich]

pyregion is an optional dependency (as I understand), so the test in test_alma_utils.py should handle it accordingly.
Right now it throws an ImportError when test are run on a setup without pyregion. I'm not sure why it isn't picked up by Travis.

======================================================================== ERRORS =========================================================================
_______________________________________________ ERROR collecting astroquery/alma/tests/test_alma_utils.py _______________________________________________
astroquery/alma/tests/test_alma_utils.py:5: in <module>
>   from pyregion.parser_helper import Shape
E   ImportError: No module named pyregion.parser_helper
============================================= 400 passed, 202 skipped, 2 xfailed, 1 error in 30.13 seconds ==============================================

[keflavich]

travis installs pyregion; this is a bug!

[coveralls]

Coverage remained the same at 65.33% when pulling fcaefeb on keflavich:issue549 into eb4ca1c on astropy:master.