Make coverage check effective

[kaplun]

Coverage is not (yet) supported by cython modules, hence all the Travis integration is currently unneeded.

Additionally .coverage is shipped with the final package on Pypi, which is wrong.

[kaplun]

As @jirikuncar points out, we can actually try to implement what is suggested in http://blog.behnel.de/posts/coverage-analysis-for-cython-modules.html