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',

package.json

@@ -18,15 +18,15 @@
   "license": "MIT",
   "homepage": "https://github.com/alphagov/notifications-admin#readme",
   "dependencies": {
-    "babel-core": "6.26.0",
-    "babel-preset-es2015": "6.24.1",
+    "@babel/core": "7.4.0",
+    "@babel/preset-env": "7.4.2",
     "diff-dom": "2.3.1",
     "govuk-elements-sass": "3.1.2",
     "govuk_frontend_toolkit": "7.2.0",
     "govuk_template_jinja": "0.24.0",
     "gulp": "4.0.0",
     "gulp-add-src": "1.0.0",
-    "gulp-babel": "7.0.0",
+    "gulp-babel": "8.0.0",
     "gulp-base64": "0.1.3",
     "gulp-concat": "2.6.1",
     "gulp-include": "2.3.1",