Major Upgrade - Prerender Latest Version

This release makes prerender-redis-cache work with the latest version of prerender.

Changes:

• Caches headers from requested page and serves them back upon cache response.
• Cleaned up code syntax/style.
• Added Status code list for cachability check (now caches more than just 200s!)

Note: This version is incompatible with older versions of prerender due to API changes.