TST: Fix Travis-CI builds. (Numpy version mismatch).
TST: Fix Travis-CI builds. (Numpy version mismatch).

Pre-Install numpy,astropy for tox tests, to avoid weird Travis issue.

Was seeing 'Numpy version build mismatch' errors, cf
https://github.com/astropy/astropy/issues/6133

Pre-installing numpy mostly works, but it seems we need to
pre-install astropy as well before Sphinx is happy.

DO YOU DO VOODOO LIKE I DO?
https://en.wikipedia.org/wiki/Voodoo_programming

timstaley/voevent-parse,timstaley/voevent-parse