Be less specific about version requirements
Try running tests on PHP 5.6 on Travis CI
Update use of deprecated Guzzle\Service\Client methods
Update usage of ResourceIterator::$nextToken
Fix usage of Guzzle\Http\Message\AbstractMessage::getHeaders()

It returns an array, not a Guzzle\Common\Collection.
Fix documentation of ModelBuilderInterface::createEmbeddedModel()

It should return a model, or an array of models.
Fix type hints in Desk\Relationship\ResponseParser
Extract common functionality out of ResponseVisitors
Add null check for Guzzle\Service\Description\Parameter::getItems()
Remove unused parameter from ModelBuilder::process()
Extract common code from ModelBuilder::process()