Compatibility with transformers 0.6

[sjakobi]

https://hackage.haskell.org/package/transformers-0.6.0.2/changelog

It seems that this will require some code changes:

src/Snap/Internal/Instances.hs:13:1: error:
    Could not load module ‘Control.Monad.Trans.Error’
    It is a member of the hidden package ‘transformers-0.5.6.2’.
    Perhaps you need to add ‘transformers’ to the build-depends in your .cabal file.
    Use -v (or `:set -v` in ghci) to see a list of the files searched for.
   |
13 | import           Control.Monad.Trans.Error         (Error, ErrorT)
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

src/Snap/Internal/Instances.hs:17:1: error:
    Could not load module ‘Control.Monad.Trans.List’
    It is a member of the hidden package ‘transformers-0.5.6.2’.
    Perhaps you need to add ‘transformers’ to the build-depends in your .cabal file.
    Use -v (or `:set -v` in ghci) to see a list of the files searched for.
   |
17 | import           Control.Monad.Trans.List          (ListT)
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[ysangkok]

@cydparser would it be possible to Address this? I can submit a PR if you'd like