Don't iterate over String in addParent

[vjpr]

#88

[blitzrk]

Before I manually applied this patch, I was getting

events.js:154
      throw er; // Unhandled 'error' event
      ^
RangeError: Maximum call stack size exceeded
    at formatPrimitive (util.js:452:25)
    at formatValue (util.js:261:19)
    at inspect (util.js:143:10)
    at exports.format (util.js:24:24)
    at Console.log (console.js:39:34)
    at addParent (/home/ben/Projects/asciidoc-blog/node_modules/css/lib/parse/index.js:584:13)
    at addParent (/home/ben/Projects/asciidoc-blog/node_modules/css/lib/parse/index.js:589:7)
    at addParent (/home/ben/Projects/asciidoc-blog/node_modules/css/lib/parse/index.js:589:7)
    at addParent (/home/ben/Projects/asciidoc-blog/node_modules/css/lib/parse/index.js:589:7)
    at addParent (/home/ben/Projects/asciidoc-blog/node_modules/css/lib/parse/index.js:589:7)

while using gulp-sass, even with three line CSS files (and no other gulp plugins). Not sure if so much recursion is normal, but this at least removes the bug's symptoms.