Cache pip wheels between runs in Travis, refs #323

.travis.yml

@@ -9,12 +9,17 @@ python:
 script:
   - python setup.py test
 
+cache:
+    directories:
+        - $HOME/.cache/pip
+
 # This defines further stages that execute after the tests
 jobs:
   include:
     - stage: deploy latest.datasette.io
       if: branch = master AND type = push
       script:
+        - pip install -U pip wheel
         - pip install .
         - npm install -g now
         - python tests/fixtures.py fixtures.db fixtures.json