Fix build failure in nokogumbo.

Nokogumbo-2.0.2 fails to build when Nokogiri-1.11.0 is installed.
Nokogiri introduced some changes that are apparently incompatible with
Nokogumbo.

ref: https://nokogiri.org/CHANGELOG.html#versioninfo-and-the-output-of-nokogiri-v

Gemfile

@@ -48,6 +48,9 @@ gem 'activerecord-hierarchical_query'
 gem 'pundit'
 gem 'mail'
 
+# locked to 1.10.9 to workaround an incompatibility with nokogumbo 2.0.2.
+gem 'nokogiri', '~> 1.10.9'
+
 group :production, :staging do
   gem 'unicorn', :platforms => :ruby
   gem 'capistrano3-unicorn'

Gemfile.lock

@@ -210,7 +210,7 @@ GEM
     mime-types-data (3.2020.0512)
     mimemagic (0.3.5)
     mini_mime (1.0.2)
-    mini_portile2 (2.5.0)
+    mini_portile2 (2.4.0)
     minitest (5.14.1)
     minitest-ci (3.4.0)
       minitest (>= 5.0.6)
@@ -233,9 +233,10 @@ GEM
     net-ssh (6.0.2)
     newrelic_rpm (6.11.0.365)
     nio4r (2.5.2)
-    nokogiri (1.11.0.rc2)
-      mini_portile2 (~> 2.5.0)
-    nokogiri (1.11.0.rc2-x64-mingw32)
+    nokogiri (1.10.9)
+      mini_portile2 (~> 2.4.0)
+    nokogiri (1.10.9-x64-mingw32)
+      mini_portile2 (~> 2.4.0)
     nokogumbo (2.0.2)
       nokogiri (~> 1.8, >= 1.8.4)
     ntlm-http (0.1.1)
@@ -456,6 +457,7 @@ DEPENDENCIES
   mock_redis
   net-sftp
   newrelic_rpm
+  nokogiri (~> 1.10.9)
   oauth2
   pg
   pry-byebug