Change so travis can cache pip installs between builds

.travis.yml

@@ -7,12 +7,7 @@ branches:
   only:
     - develop
 
-install:
-  - pip install cheetah pyopenssl==0.13.1
-
-cache:
-  directories:
-    - $HOME/.cache/pip
+cache: pip
 
 before_script:
  - chmod +x ./tests/all_tests.py

requirements.txt

+cheetah
+pyopenssl==0.13.1