ignore -f lines in pip freeze output

Newer versions of pip include the wheelhouse specification in the pip
freeze output, which then breaks the call to pip uninstall. Ignore the
line starting with "-f" to avoid the error and make wipeenv work
properly again.

Change-Id: Ibff7e40f620b08e685d5c6691576e527cbeb5510

Author: dhellmann

virtualenvwrapper.sh

@@ -1207,7 +1207,7 @@ function wipeenv {
         echo "Uninstalling packages:"
         cat "$req_file"
         echo
-        pip uninstall -y $(cat "$req_file" | sed 's/>/=/g' | cut -f1 -d=)
+        pip uninstall -y $(cat "$req_file" | grep -v '^-f' | sed 's/>/=/g' | cut -f1 -d=)
     else
         echo "Nothing to remove."
     fi