Load stacktrace middleware namespace.

To go with the new ring-server option to supply a different stacktrace
handler than the default.

See https://github.com/weavejester/ring-server/pull/21

I noticed that `:init` and `:destroy` also got some special treatment in `war.clj`, but that seemed very specific to those features. I couldn't quite figure out if or how this would fit. In my case, custom stack traces is a thing of the `:dev` profile anyway. Any thoughts?