Upgrade from guzzle 3.6 to guzzle 6.0 to remove composer 'abandoned' notifications
Since requiring in Ratchet to my project, every time I update composer dependencies or install new repositories, I am given a series of messages relating to Ratchet's use of Guzzle 3.6:

```
Package guzzle/* is abandoned, you should avoid using it. Use guzzle/guzzle instead. 
```

This can be solved by upgrading Ratchet's dependency on Guzzle to Guzzle 6. 
