Enable nightly builds

[jchodera]

@peastman: Can you enable nightli.es for nightly builds?

[peastman]

I'm not familiar with that service. What is it, and how does it differ from travis?

[jchodera]

It just triggers travis nightly.

If you have travis set up to email you when builds suddenly go from pass to fail, you'll get an email if something has broken.

This also keeps travis from auto-disabling, like it had done earlier for this repository because there was no activity for many months.