make sure resolv is required before it is used everywhere

closes #67

app/models/domain.rb

@@ -34,6 +34,8 @@
 #  index_domains_on_uuid       (uuid)
 #
 
+require 'resolv'
+
 class Domain < ApplicationRecord
 
   include HasUUID

app/models/domain/dns_checks.rb

@@ -1,3 +1,5 @@
+require 'resolv'
+
 class Domain
 
   def dns_ok?

app/models/domain/dns_verification.rb

@@ -1,3 +1,5 @@
+require 'resolv'
+
 class Domain
 
   def dns_verification_string

app/models/outgoing_message_prototype.rb

@@ -1,3 +1,5 @@
+require 'resolv'
+
 class OutgoingMessagePrototype
 
   attr_accessor :from

app/models/track_domain.rb

@@ -18,6 +18,8 @@
 #  excluded_click_domains :text(65535)
 #
 
+require 'resolv'
+
 class TrackDomain < ApplicationRecord
 
   include HasUUID

lib/postal/smtp_server/client.rb

@@ -1,3 +1,4 @@
+require 'resolv'
 require 'nifty/utils/random_string'
 
 module Postal