Fix database access caused by calling primary_key

This fixes a regression caused by the change to allow customization of
the primary key. While calling `primary_key` on the model is the proper
way to find it's pk, it caused a schema lookup which should be avoided
in class methods so things like asset precompilation can be run without
a database.

This fixes #54.

lib/acts_as_tree.rb

@@ -65,7 +65,7 @@ module ClassMethods
     #                            a custom column by passing a symbol or string.
     def acts_as_tree(options = {})
       configuration = {
-        primary_key:   primary_key,
+        primary_key:   "id",
         foreign_key:   "parent_id",
         order:         nil,
         counter_cache: nil,