Use less NPM module instead of vendored less

We're currently using nathansobo's fork, which doesn't depend on
`window` being undefined to operate in Node mode.

src/app/window.coffee

@@ -2,7 +2,7 @@ fs = require 'fs'
 fsUtils = require 'fs-utils'
 $ = require 'jquery'
 _ = require 'underscore'
-{less} = require 'less'
+less = require 'less'
 require 'jquery-extensions'
 require 'underscore-extensions'
 require 'space-pen-extensions'
@@ -142,7 +142,7 @@ window.requireStylesheet = (path) ->
 window.loadStylesheet = (path) ->
   content = fsUtils.read(path)
   if fsUtils.extension(path) == '.less'
-    (new less.Parser).parse content, (e, tree) ->
+    (new less.Parser({})).parse content, (e, tree) ->
       throw new Error(e.message, path, e.line) if e
       content = tree.toCSS()