Build with servant-0.13

[fierce-katie]

First of all, thanks for your work on this library!
We use cookie auth in our servant-based application and want to switch to servant-0.13 (which has some essential features), but servant-cookie-auth has explicit upper bound constraint for servant packages versions. There don't seem to be any breaking changes that can lead to major refactoring. Maybe it's a good idea to add another flag to control servant versions.

[tmbull]

I started looking into this today. (See #49) Updating for stack and cabal is trivial, but I'm currently having some trouble getting the CI build to pass. It seems that it is broken on master currently.

[TomMD]

We're now lagging further, with Servant 0.16 having been released. Has anyone here tried servant-auth-cookie with a recent servant?