Hash#slice doesn't exist in Ruby 2.1

I discovered while trying to find the build failures introduced by !6879 (merged) that Hash#slice doesn't exist in Ruby 2.1!

It's surprising we don't have more spec failures in Ruby 2.1 due to this:

app/controllers/projects/wikis_controller.rb:    params[:wiki].slice(:title, :content, :format, :message)
app/helpers/notes_helper.rb:    @comments_target.slice(:noteable_id, :noteable_type, :commit_id)
app/models/concerns/issuable.rb:      repository: project.hook_attrs.slice(:name, :url, :description, :homepage)
app/models/merge_request_diff.rb:  #   new_attributes[:st_commits].first.slice(:committed_date)
app/models/merge_request_diff.rb:  #   YAML.load(YAML.dump(new_attributes[:st_commits].first.slice(:committed_date)))
app/services/labels/transfer_service.rb:      params    = label.attributes.slice('title', 'description', 'color')
app/services/todo_service.rb:    user.todos.pending.where(criteria.slice(*valid_keys))
lib/api/entities.rb:        service.properties.slice(*field_names)
lib/banzai/filter/abstract_reference_filter.rb:          project_refs_cache.slice(*paths).values
lib/gitlab/data_builder/note.rb:          repository: project.hook_attrs.slice(:name, :url, :description, :homepage)
lib/gitlab/data_builder/push.rb:          repository: project.hook_attrs.slice(:name, :url, :description, :homepage,
lib/gitlab/import_export/relation_factory.rb:            finder_hash = parsed_relation_hash.slice(*finder_attributes)