Generalise constraints for InputT instances

[hvr]

I'm not sure why the code was written the way it was, but it seems we can do better by letting GHC directly reuse ReaderT's instances as well as relaxing the constraints from

instance Monad m       => Functor     (InputT m)
instance Monad m       => Applicative (InputT m)
instance Monad m       => Monad       (InputT m)

to

instance Functor     m => Functor     (InputT m)
instance Applicative m => Applicative (InputT m)
instance Monad       m => Monad       (InputT m)

This looks the same on GHC 7.8 (pre-AMP) as well as on GHC 7.10 (post-AMP)

[hvr]

fwiw, the Travis build failed due to network I/O timeout