Rename migrations timestamp to avoid collision
#25102 was merged recently and it contains a number of migrations with the exact same timestamp as some previously existing migrations in REX.
This causes an error when you upgrade a system where REX was installed before (db:migrate):
ActiveRecord::DuplicateMigrationVersionError: Multiple migrations have the version number 20150616080015.
I can't see a reason why this wouldn't happen on production too during an upgrade, even if we remove the migrations from REX, the timestamps are still saved in memory.