fix dependencies with litmus

$ bundle exec rake syntax lint metadata_lint check:symlinks check:git_ignore check:dot_underscore check:test_file rubocop
rake aborted!
LoadError: cannot load such file -- puppet_litmus/rake_tasks
/home/gitlab-runner/builds/s7TGStrC/0/adullact/puppet-gitlabrunner/Rakefile:1:in `require'
/home/gitlab-runner/builds/s7TGStrC/0/adullact/puppet-gitlabrunner/Rakefile:1:in `<top (required)>'
/home/gitlab-runner/builds/s7TGStrC/0/adullact/puppet-gitlabrunner/vendor/bundle/ruby/2.5.0/gems/rake-12.3.3/exe/rake:27:in `<top (required)>'
/home/gitlab-runner/.rbenv/versions/2.5.1/bin/bundle:23:in `load'
/home/gitlab-runner/.rbenv/versions/2.5.1/bin/bundle:23:in `<main>'
(See full trace by running task with --trace)