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

Tested for precise32 & centos6.5
The official example uses `gpgcheck=0` so I think that's how it should be http://wiki.nginx.org/Install

```
[nginx]
name=nginx repo
baseurl=http://nginx.org/packages/rhel/$releasever/$basearch/
gpgcheck=0
enabled=1
```