Update submodules: plugins (new commits)

plugins:
    6f293a1 Chris Smith <chris@dmdirc.com>
            Use the event bus for ClientOpened events.

            Add Guava as a build-time dependency for plugins.

            Gerrit: http://gerrit.dmdirc.com/2959

Change-Id: Ibda3afac0bca4aca08cef2d62330fed1d60d19ee