GC nuking context if not held in a variable

[Kenneth Ballenegger]

Doing the following:

sub = ZMQ::Context.new(1).socket(ZMQ::SUB)
sub.connect('tcp://127.0.0.1:13001')
sub.setsockopt(ZMQ::SUBSCRIBE, '')

Sometimes works, but sometimes will not work. It seems to have to do with the garbage collector trying to clear the context.

I think the gem should attempt to prevent the context from being released when there are sockets using it.