add modulo koan (Kernel.rem/2)

lib/koans/03_numbers.ex

@@ -20,6 +20,10 @@ defmodule Numbers do
     assert div(5, 2) == ___
   end
 
+  koan "What remains or: The Case of the Missing Modulo Operator (%)" do
+    assert rem(5, 2) == ___
+  end
+
   koan "Other math operators may produce this" do
     assert 2 * 2 === ___
   end

test/koans/numbers_koans_test.exs

@@ -8,6 +8,7 @@ defmodule NumbersTests do
       false,
       1.0,
       2,
+      1,
       4,
       4.0,
       false,