This removes the redundant "devel" matrix entry and refactors the config to inherit/override global environment vars.

This also bumps the ext-mongodb dependency to 1.4.0.

The MapReduce operation class will also need to call this for deciding which execute method to use in PHPLIB-316.

Ensure that the condition checks for MongoDB\BSON\Serializable and returns after recursing.

A fresh 3.4 and 3.6 server should already use its most recent FCV, so we need not explicitly set it.

Revise group() and mapReduce() info in upgrade guide

Also drop support for Mongo 2.4 (PHPLIB-282)