Add rails-6 to build matrix

gemfiles/rails_60.gemfile

@@ -0,0 +1,23 @@
+# This file was generated by Appraisal
+
+source "https://rubygems.org"
+
+git "https://github.com/rails/rails.git" do
+  gem "actionpack", "~> 6.0.0.alpha"
+  gem "railties", "~> 6.0.0.alpha"
+end
+
+group :development do
+  gem "appraisal", "~> 2.2"
+  gem "pry", "~> 0.10.1"
+  gem "pry-byebug", "~> 3.2.0"
+  gem "sdoc", "~> 0.4.1"
+  gem "telegram-bot-types", "~> 0.3.0"
+  gem "rspec", "~> 3.5.0"
+  gem "rspec-its", "~> 1.1.0"
+  gem "rspec-rails", "~> 3.5.0"
+  gem "rubocop", "~> 0.52.1"
+  gem "coveralls", "~> 0.8.2", require: false
+end
+
+gemspec path: "../"