Add raven (client for sentry).

Change-Id: I1cdfd41300e204deaa548c87e933f683f2a60d15
Reviewed-on: http://gerrit.dmdirc.com/3116
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager
Update submodules: plugins (new commits)

plugins:
    9195a8a Greg Holmes <greg@dmdirc.com>
            Remove url catcher plugin

            Gerrit: http://gerrit.dmdirc.com/3115

Change-Id: I456a9762d0363148e3ea7d876f974623c4dcc9fb