es2015 preset is deprecated, use env

[dantman]

The es2015 preset described in the README is deprecated, the env preset is now preferred:
https://github.com/gulpjs/gulp/tree/4.0#use-last-javascript-version-in-your-gulpfile

Although I'm not sure how this interacts with the fact that gulpfile.babel.js should be run with {targets: {node: 'current'}} while the things run by gulp (gulp-babel, babel-loader, etc...) should have the normal targets being used for the build.

[phated]

Updated! Thanks!