fix duplicate migration check (fix my last fix)

There was an error in [my] commit 019e2c466682d8fa6e9341a49eb2ba9ab9534601 .
I used a meaningless array access instead of actually adding values to
the $seen array for later duplicate checks.

Typical warning shown when this occurs:

    PHP Notice:  Undefined offset: 18 in /path/to/library/Akrabat/Db/Schema/Manager.php on line 265