[Buildbot] #3271: 0.8.12 breaks back compatibility with github change_hook

[Buildbot trac]

#3271: 0.8.12 breaks back compatibility with github change_hook
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Reporter:  skelly     |      Owner:
    Type:  undecided  |     Status:  new
Priority:  major      |  Milestone:  undecided
 Version:  0.8.12     |   Keywords:
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 Prior to f3a47a18eba449994c136fa9daa93a9554907c28, the github key in
 `change_hook_dialects` didn't care about the value. The docs set the value
 to `True` and we copied that example.

 When upgrading to 0.8.12, that value now matters. If it is not None, the
 value is used as is and an exception is raised on line 222 when the get
 attribute is used. For a boolean, there is no get attribute. I switched
 the value in our config to None and all is happy.

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Ticket URL: <http://trac.buildbot.net/ticket/3271>
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