pyenv global 3.6

We need pip3, and TravisCI build error says:

    The `pip3` command exists in these Python versions: 3.6, 3.6.3

Author: cdunn2001

.travis.yml

@@ -7,6 +7,7 @@
 # to allow C++11, though we are not yet building with -std=c++11
 
 #before_install: pyenv install 3.5.4 && pyenv global 3.5.4
+before_install: pyenv global 3.6
 # https://docs.travis-ci.com/user/languages/python/
 # "for Trusty, this means 2.7.6 and 3.4.3"
 install: