Enable TypeOperators to use (~) (#83)

GHC Proposal #371 requires TypeOperators to use type equality a~b.
The following lines are affected:

	src/Control/Monad/Catch.hs
	354:instance e ~ SomeException => MonadThrow (Either e) where
	357:instance e ~ SomeException => MonadCatch (Either e) where
	364:instance e ~ SomeException => MonadMask (Either e) where

The fix is to simply enable the extension.