Add Gemfile.lock back to the repo for Heroku.

Heorku seems to require the file to accept the push:

  tal:~/Projects/github-trello $ git push heroku master
  Initializing repository, done.
  Counting objects: 150, done.
  Delta compression using up to 4 threads.
  Compressing objects: 100% (91/91), done.
    Writing objects: 100% (150/150), 25.33 KiB | 0 bytes/s, done.
  Total 150 (delta 44), reused 143 (delta 40)

    -----> Ruby app detected
    -----> Compiling Ruby/NoLockfile
    !
    !     Gemfile.lock required. Please check it in.
    !

    !     Push rejected, failed to compile Ruby app

    To git@heroku.my-repo:my-repo-github-trello.git
  ! [remote rejected] master -> master (pre-receive hook declined)
    error: failed to push some refs to
    'git@heroku.my-repo:my-repo-github-trello.git'