Don't allow multiple update managers.

I think this fixes plugins not updating - the PluginManager was
using an UpdateManager while the rest of the client used a
CachingUpdateManager, which because of the way the bindings are
implemented were backed by different DMDircUpdateManagers.

Change-Id: Ic609631f7152475a0370326197fe25b82efc377e
Reviewed-on: http://gerrit.dmdirc.com/3067
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager