Use source-map-js instead of source-map

[onigoetz]

Hi,

There is a fork of [email protected] which is 4x times faster and still sync and written in JS only and compatible with Node 6+

More info about fork here: https://github.com/7rulnik/source-map-js#difference-between-original-source-map

This should improve the performance of escodegen without breaking backwards compatibility