Fix to Issue #46 - Factory.php attempting to create relationship for fields with {$class}_id

[GrandadEvans]

Factory.php was not taking into account fields that have {$class}_id as their primary key instead of id eg user_id as the primary key for the users table. - fixed and tests still pass.

All existing test passing but sorry - I'm only half way into "Laravel Testing Decoded" so not confident enough to write additional test as I get to the if (static::$isSaving and preg_match('/([A-z]+)[-_]id$/i', $field, $matches)) line (c. 330) and I'm not sure you want to set the $isSaving to true & start saving to a db.