CXX-1805 Update Ubuntu 14.04 variants to 16.04 and 16.04 variants to 18.04
CXX-1805 Update Ubuntu 14.04 variants to 16.04 and 16.04 variants to 18.04

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