Fixed failure when loading the Subst plugin. Control.Monad.CatchIO is
available as either MonadCatchIO-mtl or MonadCatchIO-transformers, so we
use the PackageImports language extension to let us pick one of them.
Merge pull request #446 from GlennS/subst-fix

Fixed failure when loading the Subst plugin. Control.Monad.CatchIO is