Update Gulp to 4.0.0

[lachieh]

This PR does a couple of things surrounding the upgrade to Gulp 4.0. This PR closes #52.

• Update the gulp package to 4.0.0
• Rename the gulpfile to gulpfile.babel.js to be able to utilize newest ECMAscript features
• Reorder and restructure gulp tasks and to better utilize the new gulp serial and parallel methods
• Remove unused build task
• Remove unused npm packages (gulp-gh-pages, gulp-load-plugins)
• Update npm lock file

I'm happy to continue optimizing, or to pull back and limit these changes a little. One thing to note is that the clean, sw and critical tasks all run with the default task now, whereas they had tobe run individually before. Even with these additions, the gulp parallel method means that it still runs just as fast (~8s before to ~5s now)

Further improvements could be splitting the gulpfile into a folder with modules and includes. (i.e. gulpfile.babel.js/index.js)

[janczizikow]

Hey @lachieh ,

Thanks a lot for this, this is great! 🎉

If you would be interested in splitting the gulpfile into separate modules, as you mentioned, just feel free to make a PR 🙂