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

Running:
gitit -f my.conf

Where my.conf contains the line:
plugins: /some/path/to/gitit/plugins/Subst.hs

I get the following error:
Ambiguous module name `Control.Monad.CatchIO': it was found in multiple packages: MonadCatchIO-transformers-0.3.1.0 MonadCatchIO-mtl-0.3.1.0

This change uses the PackageImports language extension to resolve this.