Use node: 'current' as target for babel-preset-env

This replaces the hand-rolled node version setup with a new feature that
was introduced in [email protected]

https://github.com/babel/babel-preset-env/blob/v0.0.7/CHANGELOG.md

Author: valscion

packages/babel-preset-react-app/index.js

@@ -68,7 +68,7 @@ if (env === 'test') {
       // ES features necessary for user's Node version
       [require('babel-preset-env').default, {
         targets: {
-          node: parseFloat(process.versions.node),
+          node: 'current',
         },
       }],
       // JSX, Flow