add comonad >= 4.0 to deps

Fixes this build issue I had.

```shell
Feeds.hs:15:8:
    Could not find module `Control.Comonad'
    It is a member of the hidden package `comonad-4.0'.
    Perhaps you need to add `comonad' to the build-depends in your .cabal file.
    Use -v to see a list of the files searched for.
```