Fix GHC 7.8.3: MatchGroup now has 4 args

Rather than use a pattern match, this patch opts to explicitly extract
the fields of interest using where syntax. This keeps compatibility
across GHC 7.8 releases.

Ref: https://github.com/ghc/ghc/commit/eeaea2df3fa585db503034f419c6e4331a4d8a84#diff-259092edcc59456f526cdef255c181d1L909

Closes #285.