Use bundle check for gem install (closes: #93)

  .git_setup_done was removed in profit to bundle check cmd (see log after)
  .gitlab_setup_done is keept in order to not re-run initial setup.

  git@gitlab:~/gitlab$ bundle check
  The Gemfile's dependencies are satisfied
  git@gitlab:~/gitlab$ echo $?
  0
  git@gitlab:/tmp/gitlabhq$ bundle check
  The following gems are missing
  ...
  git@gitlab:/tmp/gitlabhq$ echo $?
  1
Merge pull request #99 from sbadia/feature/bundle_check

Use bundle check for gem install (closes: #93)