pytest output on travis is too verbose

[rabernat]

I have to scroll over an immense amount of passing tests on travis before I can get to the failures. (example) This is pretty annoying.

The amount of tests in xarray has exploded recently. This is good! But maybe we should turn off --verbose in travis.

What does @pydata/xarray think?

[shoyer]

Agreed. 9000 lines of output is too much!

I think I originally turned this on to make it more obvious what the culprit is if/when tests stall, but that doesn't happen very often.