Make Halotools compatible with python 3.6 #718

Compatibility may actually be pretty close, since #717 resolved the raw string issue of docstrings with appearances of \\ in math-mode. There is (at least one) remaining failure caused by using unittest that seems to trigger a deprecation warning in pytest that it not yet fixed upstream:

DeprecationWarning: generator 'pytest_pycollect_makeitem' raised StopIteration

This issue is discussed in #717 (and the predecessor PR #648) and has also been noted in astropy/astroquery#815.

As discussed with @bsipocz it will be necessary to dig a little deeper into the root cause of this upstream failure with pytest/unittest. It would be difficult to simply stop using unittest since the TestCase class is used in several places, purposefully to limit the large memory-overhead of testing the full mock-making machinery.

I'll tag this with the v0.5 Milestone for now, though I don't want to indefinitely postpone release on account of this alone. Part of #717 is to set the build of python 3.6 on Travis to be an Allowed Failure. This choice was made to facilitate merging in bug-fixes of v0.5rc1 and avoid complicated rebasing, since the raw string issue is peppered throughout the entire package and is resolved by #717.