Fixed issues with the scss-lint config

A couple of issues were pointed out by @GaryJones including the
duplicated property rule being disabled and an incorrect StringQuotes
style. This should get everything patched up.
Replaced single quotes with double quotes

The latest version of the WP CSS Coding standards requires double
quotes rather than single. This gets us up to date. See
https://make.wordpress.org/core/handbook/coding-standards/css/#values
for more information.