Work around travis-ci/travis-ci#8363

Author: asottile

.travis.yml

@@ -16,6 +16,8 @@ matrix:
 install: pip install coveralls tox
 script: tox
 before_install:
+    # work around https://github.com/travis-ci/travis-ci/issues/8363
+    - pyenv global system 3.5
     - git --version
     - |
         if [ "$LATEST_GIT" = "1" ]; then