Use mongodb-native instead of mongodb-wrapper

Found a trivial bug from a rather necessary function in mongodb-wrapper
which made the whole module seem too unreliable for production use. Thus
switching to mongodb-native.
Don't use file storage in QA Reports exporter

Get the latest updated_at date from QA Dashboard server instead of
storing the information to a text file. Text files are quite unreliable
and using one would cause unnecessary steps when deploying with
Capistrano.