Update to buildbot 0.9.15

[rodrigc]

Test Plan

• portlint
• poudriere testport

[grembo]

@rodrigc I think this can be closed.

[asomers]

Any reason not to commit this?

[koobs]

@rodrigc If all the *_DEPENDS=<version-specs> up to date and matching their upstream counterparts in setup.py and it passes QA (poudriere, maketest in particular), go ahead.