Nginx configuration for limit_conn should be compatible with version >= 1.1.8

Nginx 1.1.8 introduced new format for its limit_conn/mod_evasive equivalent feature configuration directive, which is incompatible with all older Nginx versions. We should check for Nginx version installed (as we did in the past) to dynamically use compatible configuration, since Nginx 1.2.0 is current stable and will be included (at some point) in all supported Debian/Ubuntu LTS flavors, while 1.1.19 is already included in Precise and Squeeze backports.