Fix sourcemaps with traceur 0.0.72

Fixes #68 

@thlorenz I am not sure of the benefit of rebuilding the sourcmap and "Ignore mappings that are not from our source file".
Was trying to preserve the code from [index.js](https://github.com/thlorenz/es6ify/blob/master/index.js#L36). But I don't understand why it's needed either, probably unnecessary.