Locks source-map dependency

Fixes #436

`source-map` ~~does not follow semver~~ so the dependency should be locked down. This will 'solve' #436 until the functionality to support sourcemap 0.1.33 is added to this library.

I understand the ideal situation is to add support for 0.1.33, but we don't yet. This is a quick patch to fix all of the libraries that depend on UglifyJS2 for its source map features, which are currently broken.

Edit:

On looking into the issue more, the situation might not be exactly as described above. It seems to be a bit more complicated, as it was *technically* a bug that was being fixed. [Ref](https://github.com/mozilla/source-map/issues/99).

I'm unsure if always locking down the dependency is necessary, but I'm thinking it should be done *at least* until support for 0.1.33 is added.