Resolve dependence with guzzle-services

After a composer update we were seeing these errors:

> PHP Fatal error:  Class GuzzleHttp\\Command\\Guzzle\\GuzzleClient contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (GuzzleHttp\\Command\\AbstractClient::serializeRequest) in /var/www/mailchap/vendor/guzzlehttp/guzzle-services/src/GuzzleClient.php on line 144

The issue is with `guzzle-services` and filed here: https://github.com/guzzle/guzzle-services/issues/50

The workaround for the moment recommended by @mtdowling in https://github.com/guzzle/guzzle-services/issues/50#issuecomment-58906326 is:
> Use. 0.6 until it's updated.