Update babel-preset-es2015 to babel-preset-env

[mzgoddard]

Resolves

Pulls out the babel-preset-env change from #1104.

Proposed Changes

Replace babel-preset-es2015 with babel-preset-env.

Reason for Changes

babel-preset-es2015 is no longer maintained. If newer syntax from ES7 (ES2016) or ES8 (ES2017) is not used, babel-preset-env when updated to newer minor versions will keep transforms for used syntax up to date. Like autoprefixer used in scratch-paint and scratch-gui we can set the target browsers babel-preset-env should transform for with browserlist.

Quoted from #1104.

[rschamp]

Thanks @mzgoddard — the conflict might be from the other PR I just merged?

[mzgoddard]

@rschamp Yes. The swapped babel-preset (this PR) and removed buffer-loader (other PR) sit next to each other in package.json.