Forbid pip to look at the index

Author: fcollonval

.travis.yml

@@ -10,8 +10,8 @@ before_install:
 install: pip install pytest "jupyterlab~=1.0"
 script:
   - python setup.py sdist
-  - pip install jupyterlab_git[test] --find-links=dist --no-deps --no-cache-dir -v
-  - pip install jupyterlab_git[test] --find-links=dist -v
+  - pip install jupyterlab_git[test] --pre --no-index --find-links=dist --no-deps --no-cache-dir -v
+  - pip install jupyterlab_git[test]
   - jupyter labextension list
   - jupyter lab build
   - pytest jupyterlab_git -r ap