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)"

When trying to compress assets through the Rails 3 asset pipeline, this actually passes through the sass parser, which causes error:

    Invalid CSS after "...n-width:979px +": expected number or function, was " 1){.nav-collap..."
    /Users/pmm21/.rvm/gems/jruby-1.7.3/gems/sass-3.1.15/lib/sass/scss/parser.rb:927:in `expected'
    /Users/pmm21/.rvm/gems/jruby-1.7.3/gems/sass-3.1.15/lib/sass/scss/parser.rb:874:in `expected'
    /Users/pmm21/.rvm/gems/jruby-1.7.3/gems/sass-3.1.15/lib/sass/scss/parser.rb:856:in `expr!'
    /Users/pmm21/.rvm/gems/jruby-1.7.3/gems/sass-3.1.15/lib/sass/scss/parser.rb:736:in `term'
    /Users/pmm21/.rvm/gems/jruby-1.7.3/gems/sass-3.1.15/lib/sass/scss/parser.rb:718:in `expr'

There are other examples of such math, but I wasn't sure you would want to fix this here (since it is not being fixed at the source), so I didn't go through to find them all.