Rails 4.2 compatibility

In the latest RC of Rails 4.2, they renamed the instance variable containing the table name. This broke the ability to use the ```foreign_key``` method within ```change_table```. Then new version introduces a ```name``` method, which should (hopefully) be more stable. This patch tries to use the ```name``` method if it exists, and falls back to ```@table_name``` otherwise.

Without setting up appraisal or some such, I'm not sure there's an easy way to test this, but it's working for me with Rails 4.2.rc1.