Use Headless Chrome over PhantomJS for svg2png functionality

[backflip]

• Replaced gulp-raster with convert-svg-to-png
• Bumped Node to 8.9.1 (required for convert-svg-to-png) and updated require-dir and gulp-sass to be compatible. This means switching from npm-shrinkwrap.json to the auto-generated package-lock.json, yay!
• We could probably switch to the more common svg2png as soon as https://github.com/domenic/svg2png/pull/96 was merged