Lock cancancan to 3.5.x as 3.6 doesn't work in Ruby 2.6

Refs. CanCanCommunity/cancancan#851

Author: mshibuya

Appraisals

@@ -4,6 +4,7 @@ appraise 'rails-6.0' do
   gem 'rails', '~> 6.0.0'
 
   group :test do
+    gem 'cancancan', ['~> 3.0', '< 3.6']
     gem 'pundit', '~> 2.1.0'
   end
 

gemfiles/rails_6.0.gemfile

@@ -17,7 +17,7 @@ group :development, :test do
 end
 
 group :test do
-  gem "cancancan", "~> 3.0"
+  gem "cancancan", ["~> 3.0", "< 3.6"]
   gem "carrierwave", [">= 2.0.0.rc", "< 3"]
   gem "cuprite"
   gem "database_cleaner-active_record", ">= 2.0", require: false