Fix undefined method errors when DNS lookup fails

net-dns has a bug (https://github.com/bluemonk/net-dns/issues/46) that
causes this error to hit whenever the nameserver lookup via TCP times
out.  To fix this problem in the short term, use our fork of this
repository and pull in the fix
(https://gitlab.com/gitlab-org/gitlab-net-dns/merge_requests/2).

Closes https://gitlab.com/gitlab-org/gitlab/issues/11634

Gemfile

@@ -465,7 +465,7 @@ gem 'lograge', '~> 0.5'
 gem 'grape_logging', '~> 1.7'
 
 # DNS Lookup
-gem 'net-dns', '~> 0.9.0'
+gem 'gitlab-net-dns', '~> 0.9.1'
 
 # Countries list
 gem 'countries', '~> 3.0'

Gemfile.lock

@@ -370,6 +370,7 @@ GEM
       redis (> 3.0.0, < 5.0.0)
     gitlab-license (1.0.0)
     gitlab-markup (1.7.0)
+    gitlab-net-dns (0.9.1)
     gitlab-peek (0.0.1)
       railties (>= 4.0.0)
     gitlab-sidekiq-fetcher (0.5.2)
@@ -596,7 +597,6 @@ GEM
       mustermann (~> 1.0.0)
     nakayoshi_fork (0.0.4)
     nap (1.1.0)
-    net-dns (0.9.0)
     net-ldap (0.16.0)
     net-ntp (2.1.3)
     net-ssh (5.2.0)
@@ -1173,6 +1173,7 @@ DEPENDENCIES
   gitlab-labkit (~> 0.5)
   gitlab-license (~> 1.0)
   gitlab-markup (~> 1.7.0)
+  gitlab-net-dns (~> 0.9.1)
   gitlab-peek (~> 0.0.1)
   gitlab-sidekiq-fetcher (= 0.5.2)
   gitlab-styles (~> 2.7)
@@ -1222,7 +1223,6 @@ DEPENDENCIES
   mini_magick
   minitest (~> 5.11.0)
   nakayoshi_fork (~> 0.0.4)
-  net-dns (~> 0.9.0)
   net-ldap
   net-ntp
   net-ssh (~> 5.2)