Improve travis build speed

Author: wouterj

.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/.pip-cache/
+        - _build
+
+install: pip install --download-cache $HOME/.pip-cache --user sphinx==1.1.3
 
 script: sphinx-build -nW -b html -d _build/doctrees . _build/html
 
 branches:
   except:
     - github-comments
-