Upgrade to Rails 6.1.

* Swap out activerecord-hierarchical_query gem for some guy's patched
  version because the mainline version is incompatible with 6.1.
* Disable meta_request gem because it hangs puma on startup on 6.1.

Gemfile

@@ -40,7 +40,7 @@ gem 'puma'
 gem 'scenic'
 gem 'ipaddress_2'
 gem 'http'
-gem 'activerecord-hierarchical_query'
+gem 'activerecord-hierarchical_query', git: "https://github.com/walski/activerecord-hierarchical_query", branch: "rails-6-1"
 gem 'http-cookie', git: "https://github.com/danbooru/http-cookie"
 gem 'pundit'
 gem 'mail'
@@ -60,7 +60,7 @@ end
 group :development do
   gem 'rubocop'
   gem 'rubocop-rails'
-  gem 'meta_request'
+  # gem 'meta_request' # hangs on Rails 6.1
   gem 'rack-mini-profiler'
   gem 'stackprof'
   gem 'flamegraph'