use pyenv for py3k

Author: dschep

.travis.yml

@@ -10,9 +10,8 @@ matrix:
     - language: generic
       os: osx
       before_install:
-        - brew update
-        - brew install python3
-        - virtualenv env -p python3
+        - pyenv install 3.6.3
+        - virtualenv env -p $(pyenv root)/versions/3.6.3/bin/python
         - source env/bin/activate
     - language: python
       python: 2.7