Fix flipper_errors_raw pipeline

Summary:
It turns out puma was erroring on the GK field inside the log json, because of improperly escaped " in the fields.

To simplify it I'm changing it to use simple comma separated key:value pairs, since they are always plain string and boolean values.
"gatekeepers":"TEST_PASSING_GK:false,TEST_FAILING_GK:false,devtools_sonar_team:false"

Reviewed By: passy

Differential Revision: D13782283

fbshipit-source-id: 3f331dc9189b1a57f69a2af0d4393baf45e1fa55

src/fb-stubs/GK.js

@@ -21,6 +21,6 @@ export default class GK {
   }
 
   static serializeGKs() {
-    return JSON.stringify({});
+    return '';
   }
 }