Use an older version of Nokogiri in our test env

This is a temporary fix. See the commit diff for more information.

Gemfile

@@ -1,9 +1,26 @@
 source 'https://rubygems.org'
 
-if RUBY_PLATFORM == 'java'
-  gem 'warbler'
-end
+gem 'warbler', platforms: :jruby
+
+# FIXME:
+#
+# There's an issue in 1.12.5 that causes XHTML elements to be generated badly,
+# causing Gollum's test suite to fail.[1] The issue has been fixed upstream,
+# but we're still waiting for a new Nokogiri point release.
+#
+# However, 1.12.5 is a security patch, so we don't want end users to use an
+# older version of Nokogiri. But this is safe to do in our CI environment.
+#
+# Once there's a new Nokogiri release, we can remove this dependency and JRuby
+# CI should pass normally again.
+#
+# Note that Nokogiri 1.11+ does not support Ruby v2.4.x anymore. So to make our
+# current CI workflows pass, we should only try to install this version of
+# Nokogiri for newer Ruby versions.
+#
+# [1]: https://github.com/gollum/gollum/issues/1779
+gem 'nokogiri', '1.12.4' unless RUBY_VERSION.include? '2.4'
 
 gemspec
 
-gem 'rake', '~> 13.0'
+gem 'rake', '~> 13.0'