Switch to source-map package

Spent today tracking down an issue in browserify that turned out to be the `parse-base64vlq-mappings` package incorrectly parsing mappings.

The symptom of this problem was mappings getting their `original` and `name` keys dropped. The rest of the mappings also were also off by a couple columns and got worse the larger the file was.

Switching to mozilla's source-map package resolved the problem nicely for me.

I'm not sure if you wanted to depend on it or fix `parse-bas64vlq-mappings`, but it's here if you decide on the former.