Don't use nodes that are missing HelloResponse::NODE_HOST

[dominics]

Fixes #46

[dominics]

Not really safe for merging because of e.g.

Notice: Undefined index: b27f65e00cb76bb4de71dab8148f3a09d93251cd in /vagrant/disque-php/src/Connection/Manager.php on line 199

Call Stack:
    0.0001     350824   1. {main}() /vagrant/disque-php/test.php:0
    0.0458     615432   2. Disque\Client->ping() /vagrant/disque-php/test.php:15
    0.0458     615488   3. Disque\Client->__call() /vagrant/disque-php/test.php:15
    0.0458     615488   4. Disque\Connection\Manager->execute() /vagrant/disque-php/src/Client.php:152
    0.0458     615488   5. Disque\Connection\Manager->shouldBeConnected() /vagrant/disque-php/src/Connection/Manager.php:185
    0.0458     615488   6. Disque\Connection\Manager->isConnected() /vagrant/disque-php/src/Connection/Manager.php:404
    0.0458     615488   7. Disque\Connection\Manager->getCurrentNode() /vagrant/disque-php/src/Connection/Manager.php:148