bump sidekiq to 3.2.6

From upstream changelog,

    Deprecate delay extension for ActionMailer 4.2+ . [seuros, #1933]
    Poll interval tuning now accounts for dead processes [epchris, #1984]
    Add non-production environment to Web UI page titles [JacobEvelyn, #2004]

Debian already have 3.2.6 and I'd like to keep both versions in sync.