Have pip install use the cache.

Author: jhgg

.travis.yml

@@ -8,8 +8,8 @@ python:
 - pypy
 cache: pip
 install:
-- pip install pytest-cov coveralls flake8 import-order gevent==1.1b5
-- pip install pytest>=2.7.3 --upgrade
+- pip install --cache-dir $HOME/.cache/pip pytest-cov coveralls flake8 import-order gevent==1.1b5
+- pip install --cache-dir $HOME/.cache/pip pytest>=2.7.3 --upgrade
 - pip install -e .
 script:
 - flake8