* Update deps, fix test warnings.

* Support ns2.0

* Fix typo

* Remove ns1.1 support

* Net.Http and Net.Websockets.Client are not needed in ns2.0

* Move to net46 per volt

* Remove ns1.3 constants

* Changed Guild#DefaultChannel to resolve the first accessible channel

Resolves #776

This change is inline with hammerandchisel/discord-api-docs#329

RestGuild#DefaultChannelId is now obsolete and will throw a
NotSupportedException.

* RestGuild#DefaultChannelId will fall back to the guild ID

Adding an exception here would be a breaking change, so this was agreed
to fall back to the previous behavior, which would just return the guild
ID.