bilby-0.3.6 doesn't declare dependency on astropy

The new bilby-0.3.6 release explicitly depends on astropy in order to define the bilby.gw.prior.Cosmological class, but doesn't declare that dependency.

To reproduce:

#!/bin/bash
_tmpdir="/tmp/bilby-0.3.6"
python3.7 -m virtualenv ${_tmpdir}
${_tmpdir}/bin/python3.7 -m pip install "bilby==0.3.6"
MPLBACKEND="agg" ${_tmpdir}/bin/python3.7 -c "import bilby"

This produces the following error from python:

  File "/tmp/bilby-0.3.6/lib/python3.7/site-packages/bilby/__init__.py", line 21, in <module>
    from . import core, gw, hyper
  File "/tmp/bilby-0.3.6/lib/python3.7/site-packages/bilby/core/__init__.py", line 2, in <module>
    from . import likelihood, prior, result, sampler, series, utils
  File "/tmp/bilby-0.3.6/lib/python3.7/site-packages/bilby/core/sampler/__init__.py", line 17, in <module>
    from .pymc3 import Pymc3
  File "/tmp/bilby-0.3.6/lib/python3.7/site-packages/bilby/core/sampler/pymc3.py", line 13, in <module>
    from ...gw.likelihood import BasicGravitationalWaveTransient, GravitationalWaveTransient
  File "/tmp/bilby-0.3.6/lib/python3.7/site-packages/bilby/gw/__init__.py", line 1, in <module>
    from . import (calibration, conversion, cosmology, detector, likelihood, prior,
  File "/tmp/bilby-0.3.6/lib/python3.7/site-packages/bilby/gw/likelihood.py", line 15, in <module>
    from .prior import BBHPriorDict
  File "/tmp/bilby-0.3.6/lib/python3.7/site-packages/bilby/gw/prior.py", line 18, in <module>
    class Cosmological(Interped):
  File "/tmp/bilby-0.3.6/lib/python3.7/site-packages/bilby/gw/prior.py", line 23, in Cosmological
    name='luminosity_distance', latex_label='$d_L$', unit=units.Mpc),
NameError: name 'units' is not defined