Fixed bug where -getSnippet in clj.clj received only the step parameter.

cucumber.runtime.UndefinedStepsTracker actually calls the Backend
instances with 2 parameters--Step and FunctionNameGenerator--but
Clojure's Backend implementation only accepts one.

Instead of testing generation of Clojure Snippet with a test-only
ClojureSnippet instance, ClojureSnippetTest now tests the real
implementation in clj.clj.
Merge #650. Update History.md

Also remove obsolete comment from the Clojure backend.