CXX-1037 .mci.yml: Temporarily fall back from 'latest' clusters to 3.4
CXX-1037 .mci.yml: Temporarily fall back from 'latest' clusters to 3.4

This works around a mongo-orchestration issue
(https://github.com/10gen/mongo-orchestration/pull/229), which will be
addressed in a newer version of mongo-orchestration.

This temporary fallback change to .mci.yml will be reverted in
CXX-1037.

xdg/mongo-cxx-driver,acmorrow/mongo-cxx-driver,xdg/mongo-cxx-driver,mongodb/mongo-cxx-driver,xdg/mongo-cxx-driver,xdg/mongo-cxx-driver,acmorrow/mongo-cxx-driver,acmorrow/mongo-cxx-driver,mongodb/mongo-cxx-driver,mongodb/mongo-cxx-driver,acmorrow/mongo-cxx-driver,mongodb/mongo-cxx-driver