Modify dnsruby requirement

I have just update rspec-dns from 0.0.3 to 0.1.0. I faced on the error which I pasted on the bottom.

I have made some commits, please check them and if possible merge them.

- I want to not add `require 'dnsruby'` to spec_helper nor spec files.
- I think `require 'dnsruby'` should be `rspec-dns.rb` because users do not care about dnsruby or ruby resolv.
- I think `require 'resolv'` is not necessary anymore. 

Error:
```
  1) github.com should have the correct dns entries with {:type=>"A", :address=>"192.30.252.128"}
     Failure/Error: it { should have_dns.with_type('A').and_address('192.30.252.128') }
     NameError:
       uninitialized constant Dnsruby
     # ./spec/dns_spec.rb:7:in `block (2 levels) in <top (required)>'
```