Don't build devstack from the aged release branch

vagrant/release/devstack/Vagrantfile

@@ -32,7 +32,7 @@ if [ -n "$OPENEDX_RELEASE" ]; then
   # this can cause problems (e.g. looking for templates that no longer exist).
   /edx/bin/update configuration $CONFIG_VER
 else
-  CONFIG_VER="release"
+  CONFIG_VER="master"
 fi
 
 ansible-playbook -i localhost, -c local vagrant-devstack.yml --tags=deploy -e configuration_version=$CONFIG_VER $EXTRA_VARS