Update to LESS 2.4.0

[RobLoach]

No description provided.

[RobLoach]

Not sure why we need this tree.toCSS.

[balupton]

Was this change needed? It may be necessary to enable/disable compression

[balupton]

Are the comments here because of lesscss API changes?

[RobLoach]

@balupton The comments I made? Was reviewing my own code.

[derberg]

Hi guys, any news about this pull request? why it is not merged yet? these errors on travis do not provide any explanation

[balupton]

@derberg posted up some follow up questions for @RobLoach

Ideally, I'd have this below feature from https://bevry.me/patreon done before working on more plugins:

Abstract out the powerful DocPad Plugin System into its own Independent Plugin System.
This means taking the powerful and time-tested DocPad Plugin architecture, and abstracting it out into an independent system that can be used for any project to implement it's own plugin system easily. Existing functionality allows plugins to listen to events that can even be asynchronous which are executed safely an in prioritised order either in parallel or serially by the mediator's choosing. New features will include semantic versioning support for plugins, as well as automatic installation of the correct plugin version for your existing peer dependencies.

[balupton]

added in the now released v2.5.0