renamed manual stacking key

exceptionless/Exceptionless#187

Author: niemyjski

dist/exceptionless.js

@@ -118,7 +118,7 @@ export class EventBuilder {
      */
   public setManualStackingKey(manualStackingKey: string): EventBuilder {
     if (manualStackingKey) {
-      this.setProperty('@manual_stacking_key', manualStackingKey);
+      this.setProperty('@stack', manualStackingKey);
     }
 
     return this;