Move away from py.test in Travis .yml config

[nth10sd]

We are using py.test in .travis.yml but not in .appveyor.yml - I thought it was the same as pytest or python -m pytest but apparently they are all different.

References:

• StackOverflow - pytest is the "new hotness"
• pytest-dev #1629 - Introduce pytest command as recommended entry point
  • "It's possible that in future we will deprecate py.test and potentially even remove it."
• pytest-dev #2564 - calling pytest or python -m pytest should have identical behaviour

So it seems like we should at least move away from py.test. @jschwartzentruber what do you think it should be, pytest or python -m pytest?

[jschwartzentruber]

Standardizing on python -m pytest. @nth10sd will make a PR