Include cascade in Migration create_join_table

[jdevine]

I think the create_join_table code should automatically add "on delete cascade" to both columns' foreign key constraints. Are there instances when you would not want this on a two-column join table?

[jeremyevans]

I don't think I have this on any of my join tables. Certainly it makes some cases easier, but I don't think it's worth breaking backwards compatibility in this case.