change mgo repo

# What is this PR do?

- just refactor they way users auto follow group from update DB in a loop to user single batch update command to update the MY_GROUP collection

# Use scenario
When creating a new group, then some admins or people who's preference has the auto follow new created group will follow that new created group.


# NOTE
- Need to upgrade mongodb to 2.6+
- Use new mgo repo(http://gopkg.in/mgo.v2), the old one (labix.org/v2/mgo) should be deserted.

  So dear reviewer, please remind me to write a post to notify developers about this issue after this PR is approved:)


# Key review point
The [bulk](https://github.com/go-mgo/mgo/blob/v2/bulk.go) provided in mgo pkg, it's not matured yet but fit enough to the fix our problem. More details can be found [here](http://blog.mongodb.org/post/84922794768/mongodbs-new-bulk-api)