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.

Prior this fix, cucumber-clojure throws the following exceptions:
```
Exception in thread "main" clojure.lang.ArityException: Wrong number of args (3) passed to: clj$-getSnippet
        at clojure.lang.AFn.throwArity(AFn.java:437)
        at clojure.lang.AFn.invoke(AFn.java:47)
        at cucumber.runtime.clj.Backend.getSnippet(Unknown Source)
        at cucumber.runtime.UndefinedStepsTracker.getSnippets(UndefinedStepsTracker.java:32)
        at cucumber.runtime.Runtime.getSnippets(Runtime.java:183)
        at cucumber.runtime.snippets.SummaryPrinter.printSnippets(SummaryPrinter.java:33)
        at cucumber.runtime.snippets.SummaryPrinter.print(SummaryPrinter.java:18)
        at cucumber.runtime.Runtime.printSummary(Runtime.java:125)
        at cucumber.runtime.Runtime.run(Runtime.java:114)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
```