Only parse one parent for plugins.

We can't support multiple parents with the new DI framework,
and looking at the code it seems they never fully worked anyway.

Change-Id: I053a57b82f77e9f570efdc463f86dd6653355a3e
Depends-On: If8938f708244ed20d8d9e1c481ecb76bd2c0fac5
Reviewed-on: http://gerrit.dmdirc.com/2849
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Update submodules: plugins (new commits)

plugins:
    4252d6e Chris Smith <chris@dmdirc.com>
            Support plugin changes.

            Depends-On: I053a57b82f77e9f570efdc463f86dd6653355a3e
            Gerrit: http://gerrit.dmdirc.com/2848

Change-Id: Ic5b06afa2dfd5a57fee1179506e38c5ce49d7ce7