silence bluebird warning (close #625)

yyx990803 · yyx990803 · commit 65a0845c5239 · 2017-02-06T12:28:28.000-05:00
diff --git a/lib/style-rewriter.js b/lib/style-rewriter.js
@@ -97,6 +97,7 @@ module.exports = function (css, map) {
     .then(function (result) {
       var map = result.map && result.map.toJSON()
       cb(null, result.css, map)
+      return null // silence bluebird warning
     })
     .catch(function (e) {
       console.log(e)
