Update mongo-c-driver repository location

It appears that the MongoDB project has moved
git://github.com/mongodb/mongo-c-driver.git to
git://github.com/mongodb/mongo-c-driver-legacy.git and replaced
git://github.com/mongodb/mongo-c-driver.git with a new repository for
the next generation of the driver.  Until support for this new driver is
implemented, point the submodule at the new repository location for the
old driver.

Signed-off-by: Kevin Locke <klocke@quantpost.com>