Ruby 3.0 support, drop support for Ruby < 2.4

Gemfile

@@ -4,17 +4,17 @@ gemspec
 group :development do
   gem 'appraisal', '~> 2.2'
 
-  gem 'pry', '~> 0.10.1'
-  gem 'pry-byebug', '~> 3.2.0'
-  gem 'sdoc', '~> 0.4.1'
+  gem 'pry', '~> 0.13.1'
+  gem 'pry-byebug', '~> 3.9.0'
+  gem 'sdoc', '~> 2.0.3'
 
   gem 'telegram-bot-types', '~> 0.6.2'
 
-  gem 'rspec', '~> 3.5.0'
-  gem 'rspec-its', '~> 1.1.0'
-  gem 'rspec-rails', '~> 3.5.0'
+  gem 'rspec', '~> 3.10.0'
+  gem 'rspec-its', '~> 1.3.0'
+  gem 'rspec-rails', '~> 4.0.2'
 
   gem 'rubocop', '~> 0.52.1'
 
-  gem 'coveralls', '~> 0.8.2', require: false
+  gem 'coveralls', '~> 0.8.23', require: false
 end