devel/buildbot-slave: Update to 0.8.9

• Update to 0.8.9
• Update COMMENT
• Update mock minimum version dependency
• Add LICENSE_FILE
• Use OPTIONS helpers
• Strip trailing-slash in pkg-descr WWW: URL

Changes:

http://docs.buildbot.net/current/relnotes/index.html
http://docs.buildbot.net/current/relnotes/0.8.9.html