Fix css coverage issue if using sass variables

lib/coverage.js

@@ -189,7 +189,12 @@ module.exports = inherit({
         }.bind(this), coverageLevel.NONE);
 
         var sourceStart = getPosition(rule.position.start, opts.url, opts.map),
-            sourceEnd = getPosition(rule.position.end, opts.url, opts.map),
+            // We're using the start of the last declaration here instead of the
+            // rule.position.end, because due for example to the use of sass
+            // variables and a buggy implementation of sourcemaps in both ruby sass
+            // as well as lib sass the rule end would map to the end of the variable
+            // declaration instead of the correct end of the css rule
+            sourceEnd = getPosition(rule.declarations[rule.declarations.length - 1].position.start, opts.url, opts.map),
             // Synchronous realpath() is used because of original method is totally synchronous and recursive,
             // while the order of recursive calls is important. Making the code being asynchronous won't make
             // much benefit considering only one async call can be executed to guarantee the calling order.