port existing tests to pytest

[bdewilde]

Currently, dragnet has unittest-based tests run via nose, but we could and probably should take advantage of a more modern and maintained testing framework like pytest. This is more of a chore than an issue, but just wanted to assess buy-in before writing any code.

Tasks:

• port the tests into pytest-style
• update testing dependencies in requirements.txt
• bonus: get Travis CI running tests via pytest (side note: is the make test command in .travis.yml actually working??)

[b4hand]

I'm in favor. The Travis build was working at one point in time, but I don't know if it's been checked since the project was moved

[bdewilde]

Going to close this since PR #89 was merged, but would be great to (separately) figure out why Travis CI doesn't seem to be running.