Generated from mongodb/specifications#126

By default, Composer installs the most recent stable version and declares a requirement with the caret operator (allowing any newer versions up to but not including the next major version).

Process isolation cannot be used for test execution because ReadPreference and similar driver classes cannot be serialized (see: PHPC-850).

This should have been done in 8dcc7784.

This property was missing from 8dcc7784.