Install test dependencies so deploy can work

python tests/fixtures.py needs asgiref or it fails with an error

.travis.yml

@@ -23,7 +23,7 @@ jobs:
     - stage: deploy latest.datasette.io
       if: branch = master AND type = push
       script:
-        - pip install .
+        - pip install .[test]
         - npm install -g now
         - python tests/fixtures.py fixtures.db fixtures.json
         - export ALIAS=`echo $TRAVIS_COMMIT | cut -c 1-7`