makes the connection timeout to configurable.

In customink/rspec-dns#5 , connection timeout is fixed as `1`

In my DNS environment, timeouts occur in bad conditions.
How about this modification?

Users can use this configuration in a spec file like this, or in spec_helper.rb.

````ruby
RSpec.configuration.rspec_dns_connection_timeout = 5

describe 'github.com' do
  it { should have_dns.with_type('A').and_address('192.30.252.128') }
end

````

If you do not mind, I will continue the tasks,

- [x] make the connection timeout in have_dns.rb configurable.
- [x] change documents related this modification