Move to gulp 4

[strugee]

Ref straticjs/generator-stratic#34

[strugee]

Here's a list of repositories we need to do this for:

• [x] generator-stratic
• [ ] stratic-decorate-files
• [ ] stratic-date-in-path
• [ ] stratic-paginate-indexes
• [ ] stratic-indexes-to-rss
• [ ] stratic-posts-to-index
• [ ] stratic-filter-drafts
• [ ] gulp-attach-to-template
• [ ] stratic-default-categories
• [ ] stratic-truncate-indexes
• [ ] straticjs.github.io