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

src/koans/02_strings.clj

@@ -37,13 +37,13 @@
   (= __ (string/reverse "hello"))
 
   "Maybe you want to find the index of the first occurence of a substring"
-  (= 0 (.indexOf "hello world" __))
+  (= 0 (string/index-of "hello world" __))
 
   "Or maybe the last index of the same"
-  (= __ (.lastIndexOf "hello world, hello" "hello"))
+  (= __ (string/last-index-of "hello world, hello" "hello"))
 
-  "But when something doesn't exist, it turns up negative"
+  "But when something doesn't exist, nothing is found"
-  (= __ (.indexOf "hello world" "bob"))
+  (= __ (string/index-of "hello world" "bob"))
 
   "Sometimes you don't want whitespace cluttering the front and back"
   (= __ (string/trim "  \nhello world \t \n"))