Require tests against rails 6.0, run tests against rails 6.1.alpha

Rails 6.0 requires ruby >= 2.5, so ruby 2.4 is not tested by default
and replaced with 2.6

telegram-bot.gemspec

@@ -22,8 +22,8 @@ Gem::Specification.new do |spec|
 
   spec.required_ruby_version = '~> 2.0'
 
-  spec.add_dependency 'actionpack', '>= 4.0', '< 6.0'
-  spec.add_dependency 'activesupport', '>= 4.0', '< 6.0'
+  spec.add_dependency 'actionpack', '>= 4.0', '< 6.1'
+  spec.add_dependency 'activesupport', '>= 4.0', '< 6.1'
   spec.add_dependency 'httpclient', '~> 2.7'
 
   spec.add_development_dependency 'bundler', '> 1.16'