Executing "npx gulp lib" returns "Did you forget to signal async completion?" message

[sanak]

From #617 (comment).

$ npx gulp lib
[15:32:02] Using gulpfile ~/Projects/gtt-project/git/ol-ext/gulpfile.js
[15:32:02] Starting 'lib'...
[15:32:02] The following tasks did not complete: lib
[15:32:02] Did you forget to signal async completion?

Probably, one of the following answer fix will be necessary.
https://stackoverflow.com/questions/36897877/gulp-error-the-following-tasks-did-not-complete-did-you-forget-to-signal-async

[Viglino]

The lib script is no more used and was using an old version of gulp and didn't return a completion signal.
The last commit should solve the issue.

[sanak]

@Viglino Okay, thanks for quick fix, and sorry for wrong link in #621 PR... 🙇‍♂️
I confirmed that this issue is fixed on the commit abc9489, so I will close this.