Try using tox-travis

c.f. pytest-dev/pytest#2779

Author: matthewfeickert

.travis.yml

@@ -6,6 +6,7 @@ python:
 install:
 - sudo apt-get update
 - pip install --upgrade pip setuptools
+- pip install tox-travis
 - pip install -U --process-dependency-links -e .[develop]
 - pip install --upgrade pytest
 script: