[Tests] Nose -> pytest

[leycec]

sparse_dot_mkl is phenomenal, dramatically improving both runtime and solver accuracy (which was very surprising to us) for our use case. Because sparse_dot_mkl is phenomenal, it'd be great if the test suite could be migrated from Nose to pytest.

Sadly, Nose is officially dead:

Nose has been in maintenance mode for the past several years and will likely cease without a new person/team to take over maintainership.

But you're in luck! Pytest is a well-maintained drop-in replacement for Nose. Having just packaged sparse_dot_mkl for Gentoo Linux, I can confirm that all tests pass without modification under pytest. This means that migrating from Nose to pytest is as trivial as replacing nose with pytest in both setup.py and .travis.yml.

Thanks again for the fantastic binding, @asistradition. It can't be overstated how much of an improvement this is over the default SciPy implementation. :open_mouth: