Attempt fix for pytest on py3.5

Author: jhgg

.travis.yml

@@ -7,7 +7,8 @@ python:
 - 3.5
 - pypy
 install:
-- pip install pytest pytest-cov coveralls flake8
+- pip install pytest-cov coveralls flake8
+- pip install pytest>=2.7.3 --upgrade
 - pip install -e .
 script:
 - py.test --cov=graphql