[Buildbot] #3314: Add an "IS_NINE" constant

[Buildbot trac]

#3314: Add an "IS_NINE" constant
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Reporter:  dustin       |      Owner:
    Type:  enhancement  |     Status:  new
Priority:  major        |  Milestone:  0.9.0
 Version:  0.8.10       |   Keywords:
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 This will help people migrating configurations to add some basic
 conditionals and run the same code on 0.8.x and 0.9.x

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Ticket URL: <http://trac.buildbot.net/ticket/3314>
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