- Fixed issue with not properly handled callback
  - Thanks to [Ángel González](https://github.com/Aglezabad) for [issue #18](https://github.com/docpad/docpad-plugin-less/issues/18) and [Sebastian Domagała](https://github.com/sdomagala) for [pull request #19](https://github.com/docpad/docpad-plugin-less/pull/19)