CXX-969 appveyor.yml - pin C driver to 1.3.4
CXX-969 appveyor.yml - pin C driver to 1.3.4

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