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!

Comece por packages/@ionic/v1-toolkit/src/lib/gulp.ts e reproduza a falha com ionic serve em um projeto v1 usando o gulpfile.js relatado e o Gulp V4. Verifique como o arquivo é reconhecido, como ionic:serve:before é executado e como as tasks baseadas em promise são tratadas. O trabalho estará concluído quando o gulpfile for reconhecido e ionic serve funcionar corretamente para a configuração relatada.

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