Cannot build ES2015 without ES5 transpilation

[m-mujica]

let name = "John";
let time = "today";

export default `Hello ${name}, how are you ${time}?`

steal-tools throws an error with this code if forceES5 is set to false.

Setting sourceMaps: false gets the build working

I tracked down the problem to the traspile package which uses a very old version of estraverse when trying to generate sourcemaps.