Incorrect migration to gulp@4

[Sayan751]

I wanted to contribute to this repo, as I am using this package. However, I cannot even build the, due to the fact that the gulp tasks are still using syntax from gulp@3, whereas using gulp@4 as devDependency.

Examples:

• https://github.com/froala/aurelia-froala-editor/blob/master/build/tasks/build.js#L49-L53
• https://github.com/froala/aurelia-froala-editor/blob/master/build/tasks/doc.js#L12-L14