add pytest-astropy in the travis grid
add pytest-astropy in the travis grid

low-sky/pyspeckit,keflavich/pyspeckit,e-koch/pyspeckit,e-koch/pyspeckit,low-sky/pyspeckit,jpinedaf/pyspeckit,keflavich/pyspeckit,pyspeckit/pyspeckit,pyspeckit/pyspeckit,jpinedaf/pyspeckit