Update babel-present-env and use node: 'current' as target

Created by: valscion

This replaces the hand-rolled node version setup with a new feature that was introduced in babel-preset-env@v0.0.7: https://github.com/babel/babel-preset-env/blob/v0.0.7/CHANGELOG.md

Changes between 0.0.6 and 0.0.8 should be backwards compatible. https://github.com/babel/babel-preset-env/blob/master/CHANGELOG.md