[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    |   Keywords:
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 Now that we have our databases in version 40, we have an extra (unused)
 column in the 'builders' table.

 It should be deleted soon to make place for a clean implementation.

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