Fixed install bug re sass compile errors

[a-fro]

During site-setup.sh, and any time the styles were being compiled, I received the error:

gulp-notify: [CSS Error - Line 7] scss/dashing.scss
Error: File to import not found or unreadable: bootstrap/scss/bootstrap-flex.scss
       Parent style sheet: /Users/aaron/Vagrant/drupal-lab/web/themes/dashing/scss/dashing.scss
        on line 7 of scss/dashing.scss
>> @import "bootstrap/scss/bootstrap-flex.scss";

This update fixes the issue and allows the stylesheet to compile, though I'm not sure whether bootstrap-flex was intended to be a subset of the whole library. This commit simply includes it all (card styles and other mixins are needed, for example).