Fix for lein gorilla not working on some setups

I initially was getting `java.io.FileNotFoundException: Could not locate ring/middleware/head__init.class or ring/middleware/head.clj on classpath`

After googling a bit around toying around with __project.clj__ settings I found using later versions of __ring-json__ and __compojure__ got me to a newer exception which is referenced in the issue here https://github.com/ring-clojure/ring/issues/100 

after adding the dependency on `javax.servlet/servlet-api` I can now run `lein gorilla` with this update.