Make #precompiled_method_return_value public

 Don't port this forward to Haml 5. Use after_preamble option in
 precompiled_with_ambles instead.

 close haml#909.

 Backported from haml@ac290b5

lib/haml/compiler.rb

@@ -69,14 +69,15 @@ def precompiled_with_ambles(local_names)
       preamble + locals_code(local_names) + precompiled + postamble
     end
 
-    private
-
     # Returns the string used as the return value of the precompiled method.
     # This method exists so it can be monkeypatched to return modified values.
+    # This is called by Tilt.
     def precompiled_method_return_value
       "_erbout"
     end
 
+    private
+
     def locals_code(names)
       names = names.keys if Hash == names