Bugfix for pugTemplates.watch.tasks config

Lineman 0.38.0 switched tasks from grunt-concat-sourcemap to the more up to date grunt-contrib-concat

lineman-pug was updated around the same time but missed a remaining reference to the old concat_sourcemap:js task.

This remaining reference means that people using the latest lineman-pug on lineman 0.38.0+ were seeing an error show up in their console which said:

Running watch task
Waiting...
>> File app/templates/archive.pug changed.
Fatal error: Task concat_sourcemap:js not found.