yesod-bin/Devel.hs : Relax requirements on location of Application.hs.

I often move Application.hs to MyApp/Application.hs (and change the
module name as needed) which would result in yesod-bin printing a
warning about a missing Application.hs.

The code should now accept any file named Application.hs (either in
the top level source directory or somewhere in the tree.

I realise this may be a little controvertial, but I'm happy to discuss it further.