Revert "s/.done(/.then(/ since modern es6 track unhandled promise exceptions"

This reverts commit 09a8fec2

src/components/views/dialogs/CreateGroupDialog.js

@@ -93,7 +93,7 @@ export default createReactClass({
             this.setState({createError: e});
         }).finally(() => {
             this.setState({creating: false});
-        }).then();
+        }).done();
     },
 
     _onCancel: function() {