add .formatter.exs + format

lib/koans/04_atoms.ex

@@ -11,12 +11,12 @@ defmodule Atoms do
   koan "It is surprising to find out that booleans are atoms" do
     assert is_atom(true) == ___
     assert is_boolean(false) == ___
-    assert :true == ___
-    assert :false == ___
+    assert true == ___
+    assert false == ___
   end
 
   koan "Like booleans, the nil value is also an atom" do
     assert is_atom(nil) == ___
-    assert :nil == ___
+    assert nil == ___
   end
 end