replace compojure.handler with ring.defaults

[vhquang]

This PR updates the example to use ring.middleware.defaults instead of compojure.handler, to avoid the issue mention in:
ring-clojure/ring#251

Also update:

• clojure
• compojure
• lein-ring

[weavejester]

I think at this point there are better examples around, so this project should probably be marked as being deprecated instead.

[vhquang]

Thanks for your response. That is a fair point. Let close this PR.