fix handling of server dependency

scripts/travis_install.sh

@@ -17,7 +17,6 @@ mkdir ~/.jupyter
 pip install -q --upgrade pip
 pip --version
 pip install -e ".[test]"
-pip install -U --pre jupyterlab_server
 jlpm versions
 jlpm config current
 jupyter serverextension enable --py jupyterlab

scripts/travis_script.sh

@@ -195,8 +195,6 @@ if [[ $GROUP == usage ]]; then
     # Make sure core mode works
     jlpm run build:core
     # Make sure we have a final released version of JupyterLab server
-    pip uninstall -y jupyterlab_server
-    pip install jupyterlab_server
     python -m jupyterlab.browser_check --core-mode
 
     # Make sure we can run the built app.