Simplify HexFormat linter string searching
Simplify HexFormat linter string searching

The `HexFormat` linter was using `visit_prop` with a manual check to see
if the value of the property was a `Sass::Script::String`, rather than
just using `visit_script_string`.

Switching to `visit_script_string` simplifies the code.

Change-Id: I84a64b97071b74d9d37ff40360f4683a2e557368
Reviewed-on: https://gerrit.causes.com/29015
Tested-by: jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@causes.com>
Reviewed-by: Shane da Silva <6f6e68d1df92f30cb4b3ce35260ddf94a402f33d@causes.com>

artemartemov/scss-lint-hdm