Update libcove, and other requirements

[rhiaro]

This libcove update fixes the large blocks of JSON error reporting and oneOf validation message issues.

[jpmckinney]

@rhiaro You can use recent versions of coverage if you change Travis to run the tests with python -m coverage run …. I ran into the same problem in another repo.

nedbat/coveragepy#862 (comment)

[rhiaro]

Thanks! That didn't work for this unfortunately, due to running coveralls on travis so I'm reverting back to pinning the version for now.

[jpmckinney]

Ah, indeed, looks like a different issue.