Fix haml-lint MultilineScript offense

A false positive is triggered by the linter MultilineScript, so this
explains the switch from =~ to match?.

On top of this, since Ruby 2.4, `String#match?`, `Regexp#match?` and
`Symbol#match?` have been added.  The methods are faster than
`match`/`=~`.  Because the methods avoid creating a `MatchData` object
or saving backref. So, when `MatchData` is not used, use `match?`
instead of `match`/`=~`.