Use single quotes for code style consistency

Author: Tom Kirkpatrick

lib/middleware/user-context.js

@@ -2,7 +2,7 @@
 
 const debug = require('debug')('loopback:componenet:access:context');
 const loopback = require('loopback');
-const Promise = require("bluebird");
+const Promise = require('bluebird');
 
 module.exports = function userContextMiddleware() {
   debug('initializing user context middleware');