BLD: temporary workaround for travis numpy/py3 woes
note that py3 is now tested against numpy recent git, not pypi, just until
the travis people sort things out.

oh, the [irony](https://travis-ci.org/y-p/pandas/builds/3376439) of it all.
 That buglet popped up when I expanded Travis to run _all_ of the pandas unit tests (instead of just `pandas.tests`). I need to find a place to reproduce it and debug.
 travis fixed this on their side a while back.
