Feature/Implement not RFC MX lookup flow

Added ability to use not RFC MX lookup flow (MX and Null MX records will be checked on the DNS validation layer only).

Truemail.configure do |config|
  # Optional parameter. This option will provide to use not RFC MX lookup flow.
  # It means that MX and Null MX records will be cheked on the DNS validation layer only.
  # By default this option is disabled.
  config.not_rfc_mx_lookup_flow = true
end

1. Updated Truemail::Configuration
2. Updated Truemail::Validate::Mx
3. Updated gem development dependencies
4. Updated gem documentation
5. Updated gem version to 1.7.0