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 <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>