Fix typo in active_record/merit_generator.rb (#345)

Line #16 was referring to the migration_template source as 'add_merit_fields_to_model.rb'.
This source does not exist as the actual extension is `.erb`. This caused the task
to fail as there is no template with the name 'add_merit_fields_to_model.rb'.

[fixes #344]

lib/merit/generators/active_record/merit_generator.rb

@@ -13,7 +13,7 @@ def self.next_migration_number(path)
       end
 
       def copy_migrations_and_model
-        migration_template 'add_merit_fields_to_model.rb',
+        migration_template 'add_merit_fields_to_model.erb',
                            "db/migrate/add_merit_fields_to_#{table_name}.rb"
       end