Use "service nginx restart" instead of "reload" when nginx is inactive

Nginx will not execute the reload command in case its service is inactive.

It is safer to:

- check if Nginx syntax is correct with a `nginx -t` command
- if it is and its service is active, execute a `service nginx reload`
- if it is not active, execute a `service nginx restart`

https://github.com/admiracloud/narnia/blob/master/src/lib.js#L23