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

[Buildbot trac]

#3014: Add a migrator that reverts db version 40
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Reporter:  Ben       |       Owner:
    Type:  defect    |      Status:  new
Priority:  critical  |   Milestone:  0.9.0
 Version:  master    |  Resolution:
Keywords:            |
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Comment (by delanne):

 see PR https://github.com/buildbot/buildbot/pull/1334

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Ticket URL: <http://trac.buildbot.net/ticket/3014#comment:2>
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