Remove data type contexts which trigger these errors with ghc-7.8.3

when building alex-3.1.3.

src/Data/Ranged/RangedSet.hs:72:9:
    Illegal datatype context (use DatatypeContexts): DiscreteOrdered v =>

src/Data/Ranged/Ranges.hs:54:6:
    Illegal datatype context (use DatatypeContexts): Ord v =>