Update c3.js and c3.min.js

c3.js

@@ -320,6 +320,7 @@
             sheet = document.styleSheets[styleId];
             for (var ruleId = 0; ruleId < sheet.cssRules.length; ruleId++) {
                 rule = sheet.cssRules[ruleId];
+                if(rule.type !== CSSRule.STYLE_RULE) continue;
                 css = rule.cssText;
                 if (css.indexOf(selector) !== -1) {
                     if (css.indexOf(property) !== -1) {