[Buildbot] #3014: Add a migrator that reverts db version 40

[Buildbot trac]

#3014: Add a migrator that reverts db version 40
---------------------+--------------------
Reporter:  Ben       |       Owner:
    Type:  defect    |      Status:  new
Priority:  critical  |   Milestone:  0.9.0
 Version:  master    |  Resolution:
Keywords:            |
---------------------+--------------------

Comment (by dustin):

 Hm, that's a mess.  If we can't re-introduce the same migraiton script,
 then I'm happy to take the cheap way out and make a set of SQL statements
 or other manual-revert instructions, but in general we should try to avoid
 reverting migrations!

--
Ticket URL: <http://trac.buildbot.net/ticket/3014#comment:3>
Buildbot <http://buildbot.net/>
Buildbot: build/test automation