Fix for caching static URLs - v2

Bugfix for: The "check if the request is for part of magento" regex does not match for URLs starting with `/js/`, `/skin/` and `/media/` when a shop has enabled *Add Store Code to Urls* for all stores. This causes caching not to work for these URLs when a cookie is set.
This fix is the improved version of https://github.com/nexcess/magento-turpentine/pull/399 after the discussion there with @aheadley