Update submodules: plugins (new commits)

plugins:
    b7c845e Chris Smith <chris@dmdirc.com>
            Support for core changes.

            Depends-On: I24dfd19a887a4bafe767aa24377d6dbeb305f77d
            Gerrit: http://gerrit.dmdirc.com/3868

Change-Id: If7c52208206119b0debc9d43bb5c21f85606c087