update disconnect method for opsdroid 0.13.0 about connector-matrix

[Cadair]

I fixed this in #36

[jacobtomlinson]

After thinking about this I'm not sure the Connector API is quite right. There is no compelling reason why an opsdroid pointer should have to be passed on disconnect as the connector should already have a pointer to it stored as an attribute from when it was set up.

See opsdroid/opsdroid#725 for more information on my requested change.

Once this is implemented this issue will no longer be necessary.

[jacobtomlinson]

As the change will not be in until the next release you may wish to make the change but ensure it is an optional kwarg so that when the API changes again it will not break. You could then remove this when matrix is merged as a core connector.

[Cadair]

This can be closed.