Merge pull request #316 from jupyter/jasongrout-patch-1

Fix git clean command

Author: jasongrout

RELEASE.md

@@ -1,7 +1,7 @@
 To create a release, update the version number in jupyter_server/_version.py, then run the following:
 
 ```
-git clean -dfxx
+git clean -dffx
 python setup.py sdist
 python setup.py bdist_wheel
 export script_version=`python setup.py --version 2>/dev/null`