Travis builds failing

[perrygeo]

See https://travis-ci.org/perrygeo/simanneal/jobs/566946682 and #28

[deckar01]

I turned on Travis for my fork and the tests passed. There were a few differences in the environments. Mine defaulted to xenial and the failed build defaulted to trusty. I was able to reproduce a build error by setting the dist to trusty in the config.

The underlying issue seems to be pytest-cov's required version range for pytest being updated to >=3.6.0. The trusty image comes with pytest 3.3.0 preinstalled and pip errors instead of updating the existing install.

Hard coding the dist would temporarily fix the issue, but dependencies could get out of sync again as preinstalled packages and dependencies change. To avoid this occurring again, the best fix is probably to force pip to upgrade existing dependencies.