Add appraisals for Rails 6.1

Rails 6.1 requires an upgrade to rspec-rails 4.0.0 which fixes a
WrongScopeError: rspec/rspec-rails#2215
Rspec-rails 4.0.0 dropped support for Rails below 5.0, so Rails 4
has been removed from the appraisals.

Appraisals

@@ -1,4 +1,4 @@
-{ '4.0' => '4.0.0', '4.1' => '4.1.0', '4.2' => '4.2.0', '5.0' => '5.0.0', '5.1' => '5.1.0', '5.2' => '5.2.0', '6.0' => '6.0.0' }.each do |rails, version|
+{ '4.2' => '4.2.0', '5.0' => '5.0.0', '5.1' => '5.1.0', '5.2' => '5.2.0', '6.0' => '6.0.0', '6.1' => '6.1.0' }.each do |rails, version|
   appraise "rails-#{rails}" do
     gem "rails", "~> #{version}"
   end

awesome_nested_set.gemspec

@@ -26,7 +26,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'pry'
   s.add_development_dependency 'pry-nav'
   s.add_development_dependency 'rake', '~> 10'
-  s.add_development_dependency 'rspec-rails', '~> 3.8.0'
+  s.add_development_dependency 'rspec-rails', '~> 4.0.0'
 
   s.cert_chain = [File.expand_path('certs/parndt.pem', __dir__)]
   if $PROGRAM_NAME =~ /gem\z/ && ARGV.include?('build') && ARGV.include?(__FILE__)

gemfiles/rails_6.1.gemfile

@@ -0,0 +1,13 @@
+# This file was generated by Appraisal
+
+source "https://rubygems.org"
+
+gem "rails", "~> 6.1.0"
+
+platforms :ruby do
+  gem "sqlite3"
+  gem "mysql2", "< 0.4.0"
+  gem "pg", "0.21.0"
+end
+
+gemspec path: "../"