Use Node ES6 instead of Babel

[geertweening]

Babel was added in this commit (6888d4b).

We don't use Babel in the other Five Bells components. We rely on ES 6 support from Node 4.x and up.

While I understand it's nice to have Babel support for language features we don't have in Node, I don't think it outweighs the drawbacks:

• source code does not match the executed code, which can make it hard to debug
• Babel's transpiled code can have different behavior than Node's build in version of the same code
• additional dependencies
• Babel configuration and plugin management can be cumbersome at times

Considering the above and aiming for consistency across the five-bells-* repos I suggest removing Babel and sticking with Node ES 6 features.