Show an error if unsupported version of nginx or ngx_lua is used

[knyar]

Following #97, we could improve error handling:

1. determine minimal versions of nginx and ngx_lua that are required for this library
2. check ngx.config.nginx_version and ngx.config.ngx_lua_version and display an error during init if any of those is too low

Bonus points for adding another Docker-based test to ensure that the library can still run on the documented minimal version when changes are made.