SassC::SyntaxError: Error: Invalid CSS after " @return math"

## What should happen?
The rails assets:precompile command should compile SCSS without errors when using the foundation-sites SCSS files in a Rails project.

## What happens instead?
The precompilation fails with the following SassC error:
SassC::SyntaxError: Error: Invalid CSS after " @return math": expected expression (e.g. 1px, bold), was ".abs($a);"
on line 23:17 of node_modules/foundation-sites/scss/util/_math.scss
from line 5:1 of node_modules/foundation-sites/scss/util/_util.scss
from line 25:1 of node_modules/foundation-sites/scss/foundation.scss
from line 14:1 of app/assets/stylesheets/application.scss
>> @return math.abs($a);
----------------^

## Your Environment

- Foundation version(s) used: 6.9.0

Start by reproducing the failure with rails assets:precompile using the Foundation 6.9.0 SCSS files. Trace the import path from app/assets/stylesheets/application.scss through node_modules/foundation-sites/scss/foundation.scss, util/_util.scss, and util/_math.scss; done means the SCSS compiles without the reported SassC::SyntaxError.

Written by the indexing model from the issue text.