Get rid of the pip cache

.travis.yml

@@ -7,8 +7,8 @@ python:
   - 3.4
   - 3.5
 install:
-  - pip install -r requirements.txt --download-cache $HOME/.pip-cache
-  - pip install -r test-requirements.txt --download-cache $HOME/.pip-cache
+  - pip install -r requirements.txt 
+  - pip install -r test-requirements.txt
 script: nosetests
 after_success:
   - codecov