Avoid ajv@6.9.0 for now

Because ajv@6.9.0 does not work with webpack.
Ref: ajv-validator/ajv#941

railties/test/isolation/abstract_unit.rb

@@ -488,6 +488,22 @@ class ActiveSupport::TestCase
   Dir.chdir(app_template_path) { `yarn add webpack@4.17.1 --tilde` }
   Dir.chdir(app_template_path) { `yarn add webpack-cli` }
 
+  # FIXME: Temporary fix for webpack + ajv@6.9.0 compatible issue.
+  #        See https://github.com/epoberezkin/ajv/issues/941
+  Dir.chdir(app_template_path) do
+    package = File.read("package.json")
+    resolutions = <<~EOS
+      ,
+        "resolutions": {
+          "ajv": "6.8.1"
+        }
+      }
+    EOS
+    if package =~ /\n}\n\z/
+      File.open("package.json", "w") { |f| f.puts $` + resolutions + $' }
+    end
+  end
+
   # Fake 'Bundler.require' -- we run using the repo's Gemfile, not an
   # app-specific one: we don't want to require every gem that lists.
   contents = File.read("#{app_template_path}/config/application.rb")