Break dependency loop with optional source-map

uglifyjs -> source-map -> dryice -> uglifyjs
is a dependency loop - it is usually a very good idea to get rid of them.
It can be tedious to explain.
Maybe refer to [debian policy 7.2, paragraph 6](https://www.debian.org/doc/debian-policy/ch-relationships.html#s-binarydeps) to understand how it is bad for package management as a whole.
I propose that particular solution because dryice doesn't rely on source-map and uglifyjs runs all right without source-map (as long as we don't want to get source maps).

This might only be needed by distributions like debian, in which a separate patch can be maintained,
so i'd understand perfectly if that PR was rejected.

Regards,
Jérémy.