Update submodules: plugins (new commits)

plugins:
    526cb3e Chris Smith <chris@dmdirc.com>
            Use config interfaces where possible.

            Depends-On: Iaabedda2fa68c4e5562bb069dcdf15fa0c3beeea
            Gerrit: http://gerrit.dmdirc.com/2767

Change-Id: Ibf9f673af600c9b02ce46c00826014c0d7fcce32