Address future deprecation warnings.

Author: aki-nishimura

_sass/vendor/susy/susy/output/support/_support.scss

@@ -67,7 +67,7 @@
 
     @each $_type, $_req in $requirements {
       @each $_i in $_req {
-        $_pass: call(unquote("#{$_type}-exists"), $_i);
+        $_pass: call(get-function(unquote("#{$_type}-exists")), $_i);
 
         @if not($_pass) {
           $_fail: true;