Use Travis CI's keyword for pip caching (#334)

jdufresne · gsnedders · commit 984f934d30ba · 2017-04-19T13:01:00.000+01:00
https://docs.travis-ci.com/user/caching/#pip-cache
diff --git a/.travis.yml b/.travis.yml
@@ -9,9 +9,7 @@ python:
 
 sudo: false
 
-cache:
-  directories:
-    - $HOME/.cache/pip
+cache: pip
 
 env:
   - USE_OPTIONAL=true
