[Buildbot] #3091: default version can't be changed from the web interface

[Buildbot trac]

#3091: default version can't be changed from the web interface
----------------------------+-------------------------------
Reporter:  sa2ajj           |      Owner:
    Type:  support-request  |     Status:  new
Priority:  major            |  Milestone:  sys - on-bb-infra
 Version:                   |   Keywords:
----------------------------+-------------------------------
 After adding `0.8.10` as a new version, I tried to make it default and got
 this error message:

 {{{
 Warning: Error writing to trac.ini, make sure it is writable by the web
 server. Your changes have not been saved.
 }}}

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Ticket URL: <http://trac.buildbot.net/ticket/3091>
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Buildbot: build/test automation