Fix78

Use mtl-2.2.1.*, exceptions in haskell-cgi, builds successfully with compilers: 7.4.2, 7.6.3, 7.8.3 [1]

@cartazio please review, then I'll merge to master and then upload to hackage with major version bump.

[1] https://travis-ci.org/cheecheeo/haskell-cgi/builds/32927123