Don't log every (friggin) migration version step during unit tests

Right now any test that fails spews *all* of the migration steps at INFO
level along with the test output (assuming it did migrations). This patch
squelches the INFO-level messages from migrate, which don't add anything
useful anyway. Messages look like this:

    ...
    2014-10-10 09:51:41,967 INFO [migrate.versioning.api] 264 -> 265...
    2014-10-10 09:51:41,984 INFO [migrate.versioning.api] done
    2014-10-10 09:51:41,984 INFO [migrate.versioning.api] 265 -> 266...
    2014-10-10 09:51:42,003 INFO [migrate.versioning.api] done

Change-Id: I424bbb25c04b5e327c6d5d2ff8bad266bc7d51d0
Merge "Don't log every (friggin) migration version step during unit tests"