Upgrade bundler-audit from 0.4.0 to 0.5.0

Bundler Audit can now run the `check` command with an `—update` flag to update the Ruby CVE repository before checking. This removes the need for two separate commands in GitLab CI ([source](https://github.com/rubysec/bundler-audit#synopsis)).

See the Changelog for more information: https://github.com/rubysec/bundler-audit/blob/master/ChangeLog.md#050--2015-02-28

See merge request !3433