v1-toolkit is failing to recognize the gulpfile.js

**Description:**
I've updated gulp to V4, also the gulpfile.js.

But, the last commit is searching for plain functions, which is not a GulpV4 standard.
Anyway I've accomodated the file to accomplish what is expected, so the gulpfile.js is recognized and tasks loaded, but again is not working correctly with the promises.

**Steps to Reproduce:**
"ionic serve" is no longer working for v1 projects, with the last version of packages/@ionic/v1-toolkit/src/lib/gulp.ts ...
Even when the "gulp" command works correctly by itself.

Can you provide a small example of what is expected in the gulp file?

Why is validating for plain functions, but then it just executes "ionic:serve:before" task

Best!

Start with packages/@ionic/v1-toolkit/src/lib/gulp.ts and reproduce the failure with ionic serve in a v1 project using the reported gulpfile.js and Gulp V4. Check how the file is recognized, how ionic:serve:before is executed, and how promise-based tasks are handled. Done means the gulpfile is recognized and ionic serve works correctly for the reported setup.

Written by the indexing model from the issue text.