The Ruby driver appears to only verify the server's certificate if ssl_verify is true.

https://github.com/mongodb/mongo-ruby-driver/blob/master/lib/mongo/socket/ssl.rb#L111-L114

This enhancement request is to make certificate verification the default if only ssl=true is passed. This includes both verifying the server's cert against the system CA bundle, as well as doing RFC 2818/6125 hostname matching.