nginx 1.6 and 1.7 are live and available since yesterday when the Nginx team announced its availability. The 1.6 version now includes many new features from the old development 1.5.x mainline branch (SSL improvements, SPDY 3.1 support, cache revalidation with conditional requests, auth request module and much more).
Nginx 1.6 was forked from the current 1.5 ‘mainline’ branch, and the 1.5 branch renumbered to 1.7. From now on Nginx version 1.4.x is no longer supported, so I suggest you to update your Nginx version asap.
If you want to read more about the new versions, check out this links: