Avoid newer turbo-rails for Ruby 2.6 build

Refs. hotwired/turbo-rails#681

Author: mshibuya

Appraisals

@@ -3,7 +3,7 @@
 appraise 'rails-6.0' do
   gem 'rails', '~> 6.0.0'
   gem 'psych', '~> 3.3'
-  gem 'turbo-rails', ['!= 2.0.8', '!= 2.0.9']
+  gem 'turbo-rails', '< 2.0.8'
 
   group :test do
     gem 'cancancan', ['~> 3.0', '< 3.6']

gemfiles/rails_6.0.gemfile

@@ -7,7 +7,7 @@ gem "devise", "~> 4.7"
 gem "net-smtp", require: false
 gem "rails", "~> 6.0.0"
 gem "sassc-rails", "~> 2.1"
-gem "turbo-rails", ["!= 2.0.8", "!= 2.0.9"]
+gem "turbo-rails", "< 2.0.8"
 gem "vite_rails", require: false
 gem "webpacker", require: false
 gem "webrick"