travis with macOS fails due to sphinx python 3.5 requirement

[ttinoco]

Collecting sphinx>=1.6.5 (from numpydoc->-r requirements.txt (line 1))
Downloading https://files.pythonhosted.org/packages/89/1e/64c77163706556b647f99d67b42fced9d39ae6b1b86673965a2cd28037b5/Sphinx-2.1.2.tar.gz (6.3MB)
100% |████████████████████████████████| 6.3MB 157kB/s
Complete output from command python setup.py egg_info:
ERROR: Sphinx requires at least Python 3.5 to run.