Add support for Express 3.x

[gvilarino]

Usage of req.hostname implies express is version 4.x. In 3.x the attribute is host and thus redirection ends up going to https://undefined if not using express@4.x

[paulomcnally]

Please use https://github.com/paulomcnally/node-heroku-ssl-redirect#expressjs-3x-issue-1

Thanks