minor #5455 Improve travis build speed (WouterJ)

This PR was merged into the 2.3 branch.

Discussion
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Improve travis build speed

Commits
-------

4650da9 Fix caching
0efccb5 Fix build
64b9081 Improve travis build speed

Author: weaverryan

.travis.yml

@@ -1,16 +1,18 @@
 language: python
 
-python:
-    - "2.7"
+python: "2.7"
 
 sudo: false
 
-install: 
-    - "pip install -q -r requirements.txt --use-mirrors"
+cache:
+    directories:
+        - $HOME/.cache/pip
+        - _build
+
+install: pip install sphinx==1.1.3
 
 script: sphinx-build -nW -b html -d _build/doctrees . _build/html
 
 branches:
   except:
     - github-comments
-