Adding Facter gem to bundler config

This prevents the following error:
-----
    RAILS_ENV=production bundle exec rake db:migrate
    rake aborted!
    cannot load such file -- facter
    /usr/src/foreman/lib/tasks/reset_permissions.rake:1:in `<top (required)>'
    /usr/src/foreman/Rakefile:7:in `<top (required)>'
    (See full trace by running task with --trace)