Users who are track mentors

Users who are mentors.rb

> User.all.reject { |user| user.track_mentor.empty? }.map { |user| [user.username, user.track_mentor] }
=> [["diana", ["python"]],
 ["bob", ["ruby", "haskell"]],
 ["charlie", ["javascript", "erlang", "elixir"]],
 ["alice",
  ["clojure",
   "coffeescript",
   "elixir",
   "go",
   "haskell",
   "javascript",
   "objective-c",
   "ocaml",
   "perl5",
   "python",
   "ruby",
   "scala"]]]