Update user-context.js

using const instead of var

Author: albertolobrano

lib/middleware/user-context.js

@@ -2,7 +2,7 @@
 
 const debug = require('debug')('loopback:componenet:access:context');
 const loopback = require('loopback');
-var Promise = require("bluebird");
+const Promise = require("bluebird");
 
 module.exports = function userContextMiddleware() {
   // set current user to enable user access for remote methods