Bump Babel to 7.4.0

Requires replacing `babel-preset-es2015` for
`babel-preset-env` as Babel switched to this as of
version 6:

https://babeljs.io/docs/en/babel-preset-es2015

Note: this also moves all Babel packages to using
the scoped packages syntax, descended from the
`@babel` namespace.

https://docs.npmjs.com/about-scopes

gulpfile.js

@@ -90,7 +90,7 @@ const javascripts = () => {
     ])
     .pipe(plugins.prettyerror())
     .pipe(plugins.babel({
-      presets: ['es2015']
+      presets: ['@babel/preset-env']
     }))
     .pipe(plugins.addSrc.prepend([
       paths.npm + 'hogan.js/dist/hogan-3.0.2.js',