Dependency bump for mtl and transformers

`cabal build` succeeds with `mtl-2.2.1` and `transformers-0.4.1.0`
though not with tests enabled.  `doctest` doesn't work with
`transformers-0.4*` because it depends on GHC.
Merge pull request #6 from mdeland/update-transformers

Dependency bump for mtl and transformers