Enable pip cache in travis-ci

Author: jhgg

.travis.yml

@@ -6,6 +6,7 @@ python:
 - 3.4
 - 3.5
 - pypy
+cache: pip
 install:
 - pip install pytest-cov coveralls flake8 import-order gevent==1.1b5
 - pip install pytest>=2.7.3 --upgrade
@@ -21,4 +22,4 @@ matrix:
       script:
         - flake8
         - import-order graphql
-        - py.test --cov=graphql tests tests_py35
+        - py.test --cov=graphql tests tests_py35