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