require postgres 9.4+ for ordered set aggregate functions

public/app/plugins/datasource/postgres/query_ctrl.ts

@@ -123,6 +123,7 @@ export class PostgresQueryCtrl extends QueryCtrl {
     this.selectMenu.push(aggregates);
 
     // ordered set aggregates require postgres 9.4+
+    if (this.datasource.jsonData.postgresVersion >= 904) {
       let aggregates2 = {
         text: 'Ordered-Set Aggregate Functions',
         value: 'percentile',
@@ -132,6 +133,7 @@ export class PostgresQueryCtrl extends QueryCtrl {
         ],
       };
       this.selectMenu.push(aggregates2);
+    }
 
     let windows = {
       text: 'Window Functions',