Fix warning for deprecated Range.range?/1

lib/koans/03_numbers.ex

@@ -123,8 +123,11 @@ defmodule Numbers do
     assert 0 in range == ___
   end
 
+  def is_range?(%Range{}), do: true
+  def is_range?(_), do: false
+
   koan "Is this a range?" do
-    assert Range.range?(1..10) == ___
-    assert Range.range?(0) == ___
+    assert is_range?(1..10) == ___
+    assert is_range?(0) == ___
   end
 end