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

Tested for precise32 & centos6.5
Yeah but it's nice to be assured we are using packages from the right source, especially if it's easy.
See this section http://nginx.org/en/linux_packages.html#signatures
If we check the signature, we are only vulnerable to MITM attacks in the first download (of the key). That will be safer.