MariaDbMigrationRunner assumed "batch" column to be string, but was int

[BenVercammen]

When running the MariaDbMigrationRunner in my tests, it turned out that the batch column of the migrations table created previously was an int(11) instead of a varchar column. This PR introduces a type check while reading out the actual batch value.

Some warnings:

• I could not find any existing tests to expand, and my knowledge of the framework is too limited to try to come up with my own.
• I had a quick glance at other usages of the batch column, and it seems the MySql and Postgres MigrationRunners might potentially have the same issue?
• Also, it seems that this issue might have been present for a long time, as the CREATE TABLE IF NOT EXISTS migrations statements all are using batch integer since at least a year ago.

[dukefirehawk]

Looks good. Will do a sanity check before merging.