Skip tests that use optional dependencies if not installed

[spencerahill]

Closes #167

Still need to configure CI to use the new minimal dependencies environment.

On this pass I just used pytest.importorskip. I couldn't see any benefit of xarray's more involved route that I originally mimicked. Any thoughts on this? (see below)

[spencerahill]

Actually for pytest-catchlog, pytest.importorskip isn't a viable solution. Doing it outside the test causes all of the modules tests to be skipped. Doing it inside the test doesn't work because the caplog fixture causes an error before pytest_catchlog can be imported.

So the solution is to mimic xarray's try/except-based decorator approach.

[spencerahill]

The one failing Travis build is from an error on their side in the 3.4 env; so this should be ok pending Appveyor tests finishing.

@spencerkclark could you review this when you get a chance? Thanks!

[spencerkclark]

This looks good to me! I went ahead and restarted the Travis build for 3.5, and it looks like things cleared up. Thanks for taking care of this.

[spencerkclark]

Actually, should this get a what's new entry? I suppose one could consider this a "bug fix," considering that without this setup, the test suite wouldn't pass if one only had the minimum requirements installed.

[spencerahill]

Thanks @spencerkclark!