Switch to codecov for coverage checking.

Previously used coveralls which has been increasingly unreliable.

OK, so I think this change is 100% positive:

Codecov, like coveralls, is free for open source projects.
Though it also mistakenly reports coverage on omitted files, it is trivial to inform the webpage that a file should be omitted.
It actually understands branch coverage(!)
It actually understands the distinction between branch and PR builds.
It actually understands that travis runs multiple builds of each branch/PR
It actually discovers, without hints, where the source files are (even though the coverage file reports them as being somewhere funny)
Generally much nicer web UI
Much better documentation

project-rig / rig