enable travis caching

.travis.yml

@@ -1,5 +1,8 @@
 language: python
 sudo: false
+cache:
+  directories:
+    - $HOME/.cache/pip
 python:
   - "2.6"
   - "2.7"
@@ -10,6 +13,7 @@ env:
   - DOCUTILS=0.11
   - DOCUTILS=0.12
 install:
+  - pip install -U pip
   - pip install docutils==$DOCUTILS
   - pip install -r test-reqs.txt
 before_script: flake8