fix syntax error on 2.1 and rubocop on 2.2

Background:

Hashes {:'key': 'value'} are not valid in 2.1 but are recommended by Rubocop on 2.2. We only use those when we have a key such as weird-key, weird.key, etc...

We could disable Rubocop but it wouldn't warn us about the recommended syntax since Ruby 1.9: {key: 'value'}, which is valid for Ruby 1.9+.

Workaround 1 could be disabling Style/HashSyntax: in rubocop.yml.

Workaround 2 (tried in this MR) is to trick Rubocop using .to_sym which is effectively the same as adding the :. This would allow to keep the warning in place.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/12801