Switch to 'master' of 'mongodb' for making the mongo adapter work properly.

I think mongodb-erlang is making some big refactoring now, and because of some reason I don't know, the mongodb repo tag in rebar.config isn't working anymore, it will refer to some tag in the other branch which is not compatible to boss_db, but the master branch is still working now, so we should change to use the 'master' branch of mongodb driver.