please eliminate the requirement to use "es2015" preset

[xims]

At the moment I'm getting an error
Error: Couldn't find preset "es2015" relative to directory

es2015 had been depreciated for a while and would be great to get rid of that dependency.
(https://babeljs.io/docs/en/babel-preset-es2015)

Thanks!

[KyleAMathews]

Happy to take a PR fixing this.