Add version to show servers output

This was missing in maxctrl but it was present in maxadmin.

maxctrl/lib/show.js

@@ -34,6 +34,11 @@ const server_fields = [
         path: 'attributes.state',
         description: 'Server state'
     },
+    {
+        name: 'Version',
+        path: 'attributes.version_string',
+        description: 'Server version'
+    },
     {
         name: 'Last Event',
         path: 'attributes.last_event',