Test against 1.8 beta versions on travis-ci

* phplib-522:
  PHPLIB-522: Loosen index equality checks for GridFS

* phplib-434:
  Fix typo in index documentation
  PHPLIB-434: Add note about performance of countDocuments

* phplib-540:
  PHPLIB-540: Ensure that the WriteConcernError "errInfo" object is propagated

* phplib-539:
  PHPLIB-539: Rework client-side error logic for unacknowledged findAndModify
  Rework client-side error logic for hints in updates
  PHPLIB-539: support hint in findAndDelete operations
  PHPLIB-539: Sync CRUD spec tests
  PHPLIB-539: Allow hinting for delete
  Allow customizing the driver repository for travis-ci

* phplib-537:
  PHPLIB-537: Skip change stream tests on single node clusters
  PHPLIB-537: Allow more iterations for change stream spec tests on sharded clusters
  PHPLIB-537: "Cursor not found" error is no longer resumable
  PHPLIB-537: Use whitelist to check if a change stream is resumable

* phplib-516:
  PHPLIB-516: Append PHPLIB version information to handshake data