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
astropy/astropy#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

tox.ini

@@ -12,6 +12,7 @@ commands = py.test {posargs:-sv}
 deps =
     pytest
     coverage
+    numpy
 
 # Run coverage for py27, py35 targets:
 
@@ -35,6 +36,8 @@ basepython = python2
 whitelist_externals =
     pandoc
 deps=
+    astropy
+    numpy
     sphinx
 commands=
     pip install -r documentation/requirements.txt