Fix travis-ci config (fixes #42)

This PR updates (and adapts) the Ginga `.travis.yml` file to the one from the Astropy `package-template`.
The most important change is that now [miniconda](http://conda.pydata.org/miniconda.html) is used to install the various Pythons and dependencies.

I left a bunch of `*** TODO` markers in the `.travis.yml` file for skipped builds of things that don't work yet as-usual for Astropy affiliated packages ... I think it would be OK to address these later and merge this PR now to have the travis-ci testing back on.