Avoid double download and install for dependencies

[bsipocz]

Ideally we want to install packages in a strict order.

1. numpy
2. astropy
3. dependencies

However it seems that due to the existence of the version pin file, all the dependencies are picked up during the first step. This is suboptimal for several of our typical builds (e.g. when older astropy is required, or when the build is aborted as no numpy prerelease is available, etc.).

E.g. here is a build were only numpy needed to be installed, and then the build aborted as there is no prerelease. Then in reality everything gets installed before exiting.
https://travis-ci.org/astropy/astroquery/jobs/150718956#L685

This is an LTS astropy build, yet 1.2.1 is installed first and then downgraded:
https://travis-ci.org/astropy/astroquery/jobs/150718941#L544

It's not yet clear what is an optimal workaround. To ignore the pin file, we could use e.g. --no-pin. Maybe adding it to the first, the numpy builds would be enough. Then most of the astropy version checking can be added to the pin file, too.

@astrofrog - Do you have a better idea or preference?

[bsipocz]

This was fixed with #124