[Tests] Nose -> pytest

`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](https://docs.pytest.org).

Sadly, [Nose is officially dead](https://nose.readthedocs.io):

> 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](https://docs.pytest.org). Having just [packaged `sparse_dot_mkl` for Gentoo Linux](https://github.com/leycec/raiagent/tree/master/dev-python/sparse_dot_mkl), 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:

Update the test dependency references in setup.py and .travis.yml, replacing Nose with pytest as described in the issue. Start by inspecting those two files, then run the test suite with pytest. Done means all tests pass under pytest without modification.

Written by the indexing model from the issue text.