Won't compile on GHC 7.10 because of Data.Monoid.mempty

[Matthew Wraith]

I upgraded the resolver to LTS-4.0, which worked great after I removed the import of mempty in Ordeal.

src/Ordeal.hs:11:1: Warning:
    The import of ‘mempty’ from module ‘Data.Monoid’ is redundant

<no location info>: 
Failing due to -Werror.

[Brian McKenna]

-Werror probably shouldn't be in the cabal configuration because of things like this.

[Reid Draper]

Thanks @wraithm, @puffnfresh. Fixes both issues.