remove -v from pip install on travis

it makes way too much noise

.travis.yml

@@ -10,7 +10,7 @@ before_install:
     - npm install -g configurable-http-proxy
     - git clone --quiet --depth 1 https://github.com/minrk/travis-wheels travis-wheels
 install:
-    - pip install -v --pre -f travis-wheels/wheelhouse -r dev-requirements.txt .
+    - pip install --pre -f travis-wheels/wheelhouse -r dev-requirements.txt .
 script:
     - travis_retry py.test --cov jupyterhub jupyterhub/tests -v
 after_success: