email validator: fix undeliverable? method always returning false.

`undeliverable?` calls `smtp_enabled?` which we forgot to define. It
swallowed the error so it didn't fail but it always returned false.

test/unit/email_validator_test.rb

@@ -2,6 +2,10 @@ require 'test_helper'
 
 class EmailValidatorTest < ActiveSupport::TestCase
   context "EmailValidator" do
+    setup do
+      EmailValidator.stubs(:smtp_enabled?).returns(true)
+    end
+
     context "#undeliverable?" do
       should "return good addresses as deliverable" do
         assert_equal(false, EmailValidator.undeliverable?("webmaster@danbooru.donmai.us"))