install test dependencies with —pre

to get more warning about upcoming releases that might cause breakages

.travis.yml

@@ -34,7 +34,6 @@ before_install:
       if [[ $GROUP == js* ]]; then
         npm install -g casperjs@1.1.3 phantomjs-prebuilt@2.1.7
       fi
-    - git clone --quiet --depth 1 https://github.com/minrk/travis-wheels travis-wheels
     - |
       if [[ $GROUP == docs ]]; then
         pip install -r docs/doc-requirements.txt
@@ -50,7 +49,7 @@ before_install:
       fi
 
 install:
-    - pip install -f travis-wheels/wheelhouse file://$PWD#egg=notebook[test]
+    - pip install --pre .[test]
     - wget https://github.com/jgm/pandoc/releases/download/1.19.1/pandoc-1.19.1-1-amd64.deb && sudo dpkg -i pandoc-1.19.1-1-amd64.deb