storage: done events must be called next tick

It seems that listDir() returns synchronously (!), not sure how.
This results in the done event getting called with an error but
the EE event handlers are not setup yet.

src/storage/gcs.js

@@ -167,7 +167,7 @@ function copy(apiConfig, oldFilePath, newFilePath) {
         async.eachLimit(entries, concurrency, copyFile, done);
     }, function (error) {
         events.emit('progress', `Copied ${total} files`);
-        events.emit('done', error);
+        process.nextTick(() => events.emit('done', error));
     });
 
     return events;
@@ -207,7 +207,7 @@ function removeDir(apiConfig, pathPrefix) {
     }, function (error) {
         events.emit('progress', `Deleted ${total} files`);
 
-        events.emit('done', error);
+        process.nextTick(() => events.emit('done', error));
     });
 
     return events;