Small bug fix: index is passed to Comment node but never assigned

lib/less/tree/comment.js

@@ -4,6 +4,7 @@ var Node = require("./node"),
 var Comment = function (value, isLineComment, index, currentFileInfo) {
     this.value = value;
     this.isLineComment = isLineComment;
+    this.index = index;
     this.currentFileInfo = currentFileInfo;
     this.allowRoot = true;
 };