Fix mysql timestamps with liquibase >= 3.1.

These two tables were being created with an incorrect column type for
created/updated on mysql of timestamp without time zone. (should be datetime)
As of liquibase 3.1, the "without time zone" portion started being used and
failing due to incorrect syntax. All other mysql tables use timestamp for these
columns.

This leaves us in a bad situation where some mysql databases (stage) have been
created with incorrect column types and liquibase < 3.1.

This patch attempts to correct both problems, the original changesets are
modified to create the correct column type, and the checksum fix is
incorporated so existing mysql db's will not complain about the change.
Postgres and oracle db's do not see a changed checksum as their sql has not
actually changed.

If the changeset has already been applied incorrectly, a subsequent mysql-only
changeset will run that converts the column type to datetime. If it's already
datetime, this should have no effect.

Tested against master -> this branch for a variety of combinations of
postgres/mysql and liquibase versions, everything should be working correctly
now.
Merge pull request #499 from candlepin/dgoodwin/fix-mysql-timestamps

Fix mysql timestamps with liquibase >= 3.1.