Test ginga on travis-ci

See https://github.com/astropy/package-template/pull/20
and https://github.com/astropy/astropy/wiki/travis-ci-test-status

Currently testing ginga like this will not work because of other issues,
but it should be the goal.

You should adapt the `setup.py` from https://github.com/astropy/package-template

It will still be useful to turn on testing on travis-ci ...
for now we could e.g. simpy test that this works and then make `python setup.py test` work later:
```
python setup.py install
python -c 'import ginga'
```