Install latest nginx version from upstream repo if provided `:version` doesn't match distro provided version

Tested for precise32 & centos6.5
Huh, the wiki doesn't explicitly say anything about RHEL repo keys so I missed that. Added key URI for debian and redhat repos (Ubuntu PPA already set up to use Canonical's keyserver), check it out! (y)

BTW, isn't it technically vulnerable to MITM attacks, since the key is downloaded via http? I tried https but it just didn't serve that.