Use bare double underscores instead of atoms

lib/koans/02_strings.ex

@@ -2,38 +2,38 @@ defmodule Strings do
   use Koans
 
   koan "Strings are there to represent text" do
-    assert "hello" == :__
+    assert "hello" == __
   end
 
   koan "They can be put together" do
-    assert "hello world" == :__ <> "world"
+    assert "hello world" == __ <> "world"
   end
 
   koan "Or pulled apart when needed" do
-    assert :__ == String.split("hello world")
+    assert __ == String.split("hello world")
   end
 
   koan "Be careful, a message may be altered" do
-    assert :__ == String.replace("An awful day", "awful", "incredible")
+    assert __ == String.replace("An awful day", "awful", "incredible")
   end
 
   koan "But strings never lie about themselves" do
-    assert true == String.contains?("An incredible day", :__)
+    assert true == String.contains?("An incredible day", __)
   end
 
   koan "Sometimes you want just the opposite of what is given" do
-    assert :__ == String.reverse("ananab")
+    assert __ == String.reverse("ananab")
   end
 
   koan "Other times a little cleaning is in order" do
-    assert :__ == String.strip("  \n banana\n  ")
+    assert __ == String.strip("  \n banana\n  ")
   end
 
   koan "Repetition is the mother of learning" do
-    assert "StringStringString" == String.duplicate(:__, 3)
+    assert "StringStringString" == String.duplicate(__, 3)
   end
 
   koan "Strings can be louder when necessary" do
-    assert "LISTEN" == String.upcase(:__)
+    assert "LISTEN" == String.upcase(__)
   end
 end