Fix ghc-7.8's AMP warning

Prepare for ghc-7.10 Functor-Applicative-Monad-Proposal(AMP).
see also
 http://www.haskell.org/haskellwiki/Functor-Applicative-Monad_Proposal#Missing_superclasses
for more information.

This PR tested on ghc-7.6.3 and ghc-7.8.3.