Returned empty promises in ghost-server

- I have no idea why this is required and I really don't like it
- this change looks superfluous but I had problems with Ghost not
  displaying shutting down messages properly and this is the only
  thing that got it working
- the async-awaits were getting stuck in `await this._cleanup()`,
  and then wouldn't enter into the `finally` block
- oddly, this was only reproducible when a job was running, it would
  shut down normally otherwise
- if I find the solution in the mean time, I'll happily get rid of this

core/server/ghost-server.js

@@ -169,6 +169,9 @@ class GhostServer {
             this.httpServer = null;
             this._logStopMessages();
         }
+
+        // TODO: investigate why this is required (see commit)
+        return Promise.resolve();
     }
 
     /**
@@ -203,6 +206,9 @@ class GhostServer {
         // Wait for all cleanup tasks to finish
         await Promise
             .all(this.cleanupTasks.map(task => task()));
+
+        // TODO: investigate why this is required (see commit)
+        return Promise.resolve();
     }
 
     _onShutdownComplete() {