Update tests to support PyTest 4.0 +

[smmaurer]

Some of our unit test syntax has been deprecated and removed in recent versions of PyTest.

For example, tests that directly called the df() fixture defined in test_mnl.py#L82 were raising errors when I worked on PR #222.

This shouldn't be too hard to fix, but will require going through and setting up some of the test data in alternative ways. It doesn't seem especially high-priority, though.

For now, I've pinned pytest at v3.10 in the Travis and AppVeyor scripts.