Don't use --download-cache argument

.travis.yml

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