Fix Travis-CI integration to use conda

h/t to @rhattersley who figured this out for pyugrid/pyugrid#44

Let's see how travis handles this PR!
Hmmm, you're right, `python-dateutil` isn't in the "defaults" channel. The package has been named "dateutil" instead.

But as I understand it, the "defaults" channels is *intended* to be created from the [conda-recipes](https://github.com/conda/conda-recipes) repo, and there the package **is** called "[python-dateutil](https://github.com/conda/conda-recipes/tree/master/python-dateutil)" as one would expect. I'll raise an issue with Continuum...