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.