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, which are currently broken.