Unfortunately running coverage means our tests need more than a minute longer
to run.
We still run it in the following scenarios:
- Full 'tox' run
- On Travis CI on Linux
- On the buildbot on Archlinux
But not anymore in the following scenarios:
- When running 'tox -e py35' (or py34)
- On Travis CI on OS X
- On AppVeyor
- On the buildbot except on Archlinux