fix "undefined local variable or method `source_address_inet6' "

when trying to new a resolver, 

like, 
```ruby
res = Net::DNS::Resolver.new
```
will cause a error 

```bash
"undefined local variable or method `source_address_inet6' for #<Net::DNS::Resolver:0x007fc124a406b8>"
```

that's because this [commit](https://github.com/jamesotron/net-dns/commit/61b8e2e54563cf4dcb8ed73ad27958f751f346ea) added source_address_inet6 as option but forget to implement a reader, when method "state" in resolver trying to run the following 
```ruby
str << "#{key}: #{eval(key.to_s)} \t"
```

this error will be caused.

and also bump the version to 7.2.

thanks.