Upgrade to gulp 4 and update others gulp plugins

[antleblanc]

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

This pull request was inspired by both:

• Issue Upgrade gulp to 4.0 and upgrade gulpfile to be compatible with new version html5-boilerplate#2150
• Pull request: upgrade to gulp 4 and update others dev dependencies html5-boilerplate#2151

[roblarsen]

Thanks! I'll take a look at this tomorrow.

[roblarsen]

Sorry it took so long. Testing was a little bit of an npm dance getting gulp to actually upgrade. Looks good to me. Thanks so much.