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".
I guess I was trying to go for backward compatibility. But also because this is a Browserify plugin, we need to have some control over how the sourcemaps are generated, so that we can pass them to Browserify the way its being expected.