Update poet to support Express 4

@AdamBInfinity @jsantell Wouldn't it be better to go with `^4.0.0`? (Or `>=4.0.0 <5.0.0`, if you'd prefer.) Simply the preemptive equivalent of [the commit where `>=3.2.4` was changed to `>=3.2.4 <4.0.0`](https://github.com/jsantell/poet/commit/57eba690d1a4e1b07a1d1c2b6fbb82e7c062cdca). :)