Customizing test header version information

[bsipocz]

Removing version for unused packages, and adding astropy version.

[coveralls]

Coverage remained the same at 66.6% when pulling 46f3996 on bsipocz:test_header_customization into 54e6899 on astropy:master.

[keflavich]

conftest.py is inherited from package-template, so I don't really know what this does. Can you explain it a little more?

[bsipocz]

@keflavich - The PYTEST_HEADER_MODULES dictionary was introduced in astropy v1.0 (in astropy/astropy#3157) to allow affiliated packages to add the version number of their extra dependencies to the list in the header prints when test are run.
For astroquery it looks like this:

Numpy: 1.9.1
Scipy: 0.15.0
Matplotlib: 1.5.x
h5py: not available

Some of these are irrelevant as the package is not using them, while e.g. astropy is missing. With this PR:

Numpy: 1.9.1
astropy: 1.1.dev12025