Support placeholders in DocumentsMatchConstraint and allow CommandExpectations to ignore extra events.

Also removes abstract from base spec test class.

Spec tests synced with mongodb/specifications@a1c9d3c5a7b938f595bb2547c07a238da21263f3

Spec tests synced with mongodb/specifications@4820c7bad9b06b71f169c0d4b99aba6fbb319243

PHPLIB-429: Remove support for PHP 5.5

Mark incomplete transaction tests to be fixed before 4.2-compat release.

Changes JSON decoding of spec tests to preserve array/object types.

Bump driver version on Travis CI to include libmongoc 1.14 and PHPC-1373.

Spec tests synced with mongodb/specifications@83a0e6b56bfd73b471c1b740190dbbb7c52ffa92

Additionally, force 14.04 when testing server 3.0.x since it is not available on 16.04

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.