Tell pip to use explicit download cache

.travis.yml

@@ -7,6 +7,8 @@ python:
 
 # Executed for 3.5 AND 3.5 as the first "test" stage:
 script:
+  - pip install -U pip wheel
+  - pip install --download-cache /home/travis/.cache/pip .
   - python setup.py test
 
 cache:
@@ -19,8 +21,7 @@ jobs:
     - stage: deploy latest.datasette.io
       if: branch = master AND type = push
       script:
-        - pip install -U pip wheel
-        - pip install .
+        - pip install --download-cache /home/travis/.cache/pip .
         - npm install -g now
         - python tests/fixtures.py fixtures.db fixtures.json
         - export ALIAS=`echo $TRAVIS_COMMIT | cut -c 1-7`