Update java string methods to clojure functions

With Clojure 1.8.0 finally those string methods now have corresponding
Clojure functions.

Of note here is that the clojure.string/index-of function responds
with nil if nothing is found and not -1 as .IndexOf previously did.

resources/koans.clj

@@ -24,7 +24,7 @@
                 "olleh"
                 "hello"
                 13
-                -1
+                nil
                 "hello world"
                 true
                 false