This commit changes the `publish` method to use a queue to wait for
Puback packets rather than polling a hash.  Every time the read loop
gets data or a timeout from `IO.select`, it will send a message to
everyone waiting for a `Puback` packet.  If the we're within the
deadline, then the loop executes again, if we got a packet, we'll return
the packet, and if we're outside the deadline, a `-1` is returned.

This upside is that this patch speeds up the publish method by over
100x.  Here is the benchmark:

```ruby
require "securerandom"
require "mqtt"
require "benchmark/ips"
require "stackprof"

client = MQTT::Client.new(username: 'testuser', password: 'testpasswd', client_id: "client_#{SecureRandom.hex(10)}", host: '127.0.0.1')
client.connect

Benchmark.ips do |x|
  x.report("send message") {
    i = rand(1..10)
    topic = "to/timebox#{i}/cameras"
    msg = "message #{Time.now} for timebox#{i}"
    client.publish(topic, msg, true, 1)
  }
end
```

Before this patch:

```
$ ruby -I lib thing.rb
Warming up --------------------------------------
        send message     8.000  i/100ms
Calculating -------------------------------------
        send message     85.042  (± 4.7%) i/s -    432.000  in   5.089261s
```

After this patch:

```
$ ruby -I lib thing.rb
Warming up --------------------------------------
        send message   915.000  i/100ms
Calculating -------------------------------------
        send message      9.453k (± 4.5%) i/s -     47.580k in   5.043716s
```

The downside is that the timeout isn't exact.  Since `IO.select` times
out every `0.5` seconds (according to the `SELECT_TIMEOUT` constant),
the deadline in the `publish` method could be missed by that amount of
time.

Refs njh#115

Removed older rubies, no-longer in rvm