mongodb 1.6.0alpha2

** Bug
* [PHPC-1331] - Fix typos in ReadPreference::__set_state() errors
* [PHPC-1332] - PHP array API downcasts 64-bit integers to 32-bit values on Windows
* [PHPC-1341] - Remove XFAIL for unacknowledged write concern test

** Epic
* [PHPC-837] - Update PHP driver to test in Evergreen
* [PHPC-1329] - Fix Windows build failures on AppVeyor

** Task
* [PHPC-691] - Test that Server command/query execute methods send read preference to mongos
* [PHPC-857] - Create Evergreen matrix for various test configurations
* [PHPC-858] - Add authentication axis to Evergreen matrix
* [PHPC-859] - Add SSL axis to Evergreen matrix
* [PHPC-860] - Add PHP runtime axis to Evergreen matrix
* [PHPC-861] - Add MongoDB version axis to Evergreen matrix
* [PHPC-862] - Add Evergreen matrix to run against nightly server builds
* [PHPC-863] - Add RHEL 7.1/Power 8 to the operating system axis
* [PHPC-864] - Add storage engines to Evergreen matrix
* [PHPC-1180] - Fix peer certificate verification errors for SSL clusters
* [PHPC-1240] - Remove noprealloc, nssize and smallfiles from MO configurations
* [PHPC-1271] - Add (only) RHEL on zSeries to Evergreen Matrix
* [PHPC-1276] - Create tool chain for PHP binaries
* [PHPC-1299] - Add Ubuntu 16.04/ARM64 to the operating system axis
* [PHPC-1333] - Revert to virtualized Travis CI environments
* [PHPC-1345] - Revise fragile session ID assertions
* [PHPC-1364] - Upgrade libmongoc to 1.14.0
* [PHPC-1370] - Revise test assertion for unknown modifier error message

** Improvement
* [PHPC-1301] - php_phongo_cursor_t.got_iterator should use bool type
* [PHPC-1307] - Exception interface should extend Throwable on PHP 7+
* [PHPC-1373] - WriteResult is inaccessible if executeBulkWrite() encounters socket error