Add parenthesis to scope variable math correctly

This is the same problem that twbs declined to fix upstream:
https://github.com/twbs/bootstrap/pull/8450

It raises an error when compressing assets because without the
parenthesizes, uses of @navbarCollapseDesktopWidth are evaluated
 as "@media (max-width: 979px + 1)" instead of "@media (max-width: 980px)"
Merge pull request #658 from patrickmcelwee/fix_variable_math

Add parenthesis to scope variable math correctly