Stop trying to run PHP 5.6 tests on Travis CI

Mockery has issues on PHP 5.6. The package should still be compatible,
but it can't be tested until Mockery is updated.

Revert "Try running tests on PHP 5.6 on Travis CI"

This reverts commit a52209931298cf0656545947b5cbda8b6f4697ab.
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()