BUG: common.is_hashable returns False for np.float64 on Python 3
xref #8929
xref #9193

Here are [some test results](https://travis-ci.org/shoyer/pandas/builds/47184451) on Travis-CI illustrating the issue and my [commit](https://github.com/shoyer/pandas/commit/f8eb1c687164274a6b9a9640e565de60293ad2c1)

Any ideas for the fix? This is quite likely a numpy bug, but it's one we need to work around. The simplest thing would be revert back to just calling `hash`, but I expect that would cause some other test cases to fail.

CC @aevri
