Fix typo in rhino. Fixes #896

lib/less/rhino.js

@@ -3,7 +3,7 @@ var name;
 function loadStyleSheet(sheet, callback, reload, remaining) {
     var endOfPath = Math.max(name.lastIndexOf('/'), name.lastIndexOf('\\')),
         sheetName = name.slice(0, endOfPath + 1) + sheet.href,
-        contents = sheets.contents || {},
+        contents = sheet.contents || {},
         input = readFile(sheetName);
         
     contents[sheetName] = input;