Use case-insensitive matches for tasks

Valid task list completion can be recorded as [x] or [X]. The former is more common, as it's what happens when you click the checkbox in the UI, but the latter is valid too and can come from a manual edit.

The merge request validator should consider both as checked tasks. Otherwise, it's wrong, and the error messages are very confusing.

See https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/3072 for an example.