Ensure the command document is non-empty before attempting to access the command and collection names.

Use MultipleIterator::MIT_NEED_ANY to ensure iteration doesn't stop prematurely if one collection contains fewer documents.

Use a Trusty image for PHP 5.5, since it is not available on Xenial.

PHPLIB-368: Require ext-mongodb ^1.6

#591 began using new transaction APIs, which first appeared in 1.6.0alpha1

This bumps the PHPC requirement to 1.5.2+, since that includes libmongoc 1.12 and CDRIVER-2744.

Generated from mongodb/specifications@76125be57b4738325def2064248946a834d09233