[Buildbot] #3358: Require Twisted<15.4.0 for Python-2.6

[Buildbot trac]

#3358: Require Twisted<15.4.0 for Python-2.6
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Reporter:  dustin    |      Owner:  dustin
    Type:  task      |     Status:  new
Priority:  critical  |  Milestone:  0.9.0
 Version:            |   Keywords:
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 Until we, too, drop 2.6 support, we need to be sure not to try to use
 Twisted-15.4.0 or higher.

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Ticket URL: <http://trac.buildbot.net/ticket/3358>
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Buildbot: build/test automation