Replace DB migration foreign_keys calls with foreign_key_exists?
In Rail 4.2 and higher, where native foreign key support replaces Foreigner, the "foreign_keys" call inside a DB migration should be avoided as it will throw an exception on databases (SQLite) that don't support FKs.
In Foreigner, it returned
 on SQLite. In #13244 (Rails 4.2) I'll add a compatibility layer with a deprecation warning, but our migrations that use this to check whether a key exists should just use the
foreign_key_exists? method for better compatibility.