Request "server-time" capability instead of just the old znc vendor prefixed variants

[dequis]

https://github.com/psychs/limechat/blob/master/Classes/IRC/IRCClient.m#L3677

[self send:CAP, @"REQ", @"znc.in/server-time", nil];
[self send:CAP, @"REQ", @"znc.in/server-time-iso", nil];

server-time is missing from there, which is what newer ZNC versions use since ircv3.2 was declared stable.

The spec is the same thing as znc.in/server-time-iso, so just adding it to the list of CAP REQ should be enough.

(The right thing to do would be to use CAP LS and send a single CAP REQ with space-separated capabilities that the server claims to support and the client is interested in, but that would require more refactoring. So just requesting server-time is fine too)

(Ref: #168, #169)