Since the mainline release (currently 1.11.19) has all of the latest features, you'll need to install it directly from the NGINX repositories. Thankfully, NGINX is kind enough to provide Red Hat Enterprise Linux (RHEL), CentOS, SUSE Linux Enterprise Server (SLES), Debian, and Ubuntu repositories, as well as OS X and Windows binaries.
Mainline versus stable
The stable and mainline branches don't necessarily reflect system stability, but configuration and module integration stability. Unless you have third-party integration which requires the stable release, we highly recommend the mainline release.