Add .travis.yml file to package-template

This was proposed and discussed here: https://github.com/astropy/package-template/issues/14

See travis build here: https://travis-ci.org/cdeil/package-template

I think the most important thing is to test against the latest stable astropy release and against the current astropy development version (i.e. astropy git master).

@eteq @keflavich Please comment on these questions and let me know if you think I should make other changes:
* Should we use more numpy and scipy versions?
* Should we use https://github.com/astropy/wheels ?
* Should we run `cd docs; make html`?