Update submodules: plugins (new commits)

plugins:
    4c68c5e Chris Smith <chris@dmdirc.com>
            Enable PMD for plugins.

            Fix assorted PMD errors.

            Gerrit: http://gerrit.dmdirc.com/3940

Change-Id: Ic2d5adc98512b6c6c4e99a9cfab63d7405a3b57e