Currently, the Exploration Mission Master achievement is bugged.
If you purchased a treasure map prior to 6.1 (and maybe even post 6.1) you will not be able to get the rare follower missions that give that map thereby making it impossible to complete the achieve and get Harrison Jones.
I've already reported it via in game, forums, and twitter.
- - - Updated - - -
Looks like it is being hotfixed already and getting Harrison Jones is now only going to require that you do the 6 missions he offers daily.