Fix minor jslint issue (#1697)

* Undefined propType in react component `ColumnSettings`.
  Add proper PropTypes definition.

app/assets/javascripts/components/features/notifications/components/column_settings.jsx

@@ -27,8 +27,9 @@ const ColumnSettings = React.createClass({
 
   propTypes: {
     settings: ImmutablePropTypes.map.isRequired,
+    intl: ImmutablePropTypes.object.isRequired,
     onChange: React.PropTypes.func.isRequired,
-    onSave: React.PropTypes.func.isRequired
+    onSave: React.PropTypes.func.isRequired,
   },
 
   mixins: [PureRenderMixin],