disallow multiple occurrences of the same version

If two migrations exist, say, `005-AddTable.php` and `005-AddIndex.php`, Akrabat will execute the first one returned by `dir()->read()`, and ignore the second. This may cause confusion if two devs accidentally created migrations with conflicting version numbers and the problem wasn't noticed before the database was upgraded to version 5.

This patch prevents accidental creation of multiple upgrade scripts with the same version number.