Updated to new sourceMappingURL syntax

The upstream specification has changed since #44 was closed from //@ to //# which avoids IE conditional comment compatibility issues and thus the need to wrap the declaration in a /* */ comment.

See:
    https://github.com/mishoo/UglifyJS2/pull/213#issuecomment-18215308
    https://bugzilla.mozilla.org/show_bug.cgi?id=870361