Improve coverage / coveralls integration

[bgrant]

In particular, coverage does a poor job of detecting when we run things on the engines.

https://coveralls.io/files/244315634

Also, coveralls.io's reports seem to lag our GitHub repository by about a month.

[bgrant]

I looked at this briefly, and there didn't seem to be an easy way to fix our coverage results.

Also, Coveralls.io seems to be very laggy (the posted coverage results often lag a month behind master) so I removed the badge from the README in #577.

[bgrant]

We've switched to using codecov, so this is out-of-date. Closing.