Add cli to package installs (#4274)

* Add cli to package installs

adds an optional call to the command line to run invoke commands easiert

* Add libffi7

needed for ffi / cli operations

Procfile

@@ -1,2 +1,3 @@
 web: env/bin/gunicorn --chdir $APP_HOME/InvenTree -c InvenTree/gunicorn.conf.py InvenTree.wsgi -b 0.0.0.0:$PORT
 worker: env/bin/python InvenTree/manage.py qcluster
+cli: env/bin/python -m invoke