Relax lens lower bound

servant-multipart-0.11.3 depends on `lens >= 4.17 && < 4.18`, while the previous version depended on `lens >= 4.0 && < 4.18`.

servant-multipart uses four functions from `Control.Lens`, and looking at the lens changelog none of them seem to have recent changes to behaviour. Can the lower bound please be relaxed, and a metadata revision pushed to hackage?

(Among other things, this is currently breaking installing servant-multipart via nixpkgs on darwin.)

Locate the package metadata for servant-multipart and inspect its dependency declaration for lens. Compare the four Control.Lens functions with the lens changelog, then update the lower bound and verify that a metadata revision can be pushed to Hackage; done means older compatible lens versions are accepted and the Darwin installation issue is addressed.

Written by the indexing model from the issue text.