Update rubocop, add appraisal gem

gemfiles/rails_42.gemfile

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+# This file was generated by Appraisal
+
+source "https://rubygems.org"
+
+gem "actionpack", "~> 4.2"
+gem "railties", "~> 4.2"
+
+group :development do
+  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", "~> 0.51.0"
+  gem "coveralls", "~> 0.8.2", require: false
+end
+
+gemspec path: "../"

gemfiles/rails_50.gemfile

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+# This file was generated by Appraisal
+
+source "https://rubygems.org"
+
+gem "actionpack", "~> 5.0"
+gem "railties", "~> 5.0"
+
+group :development do
+  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", "~> 0.51.0"
+  gem "coveralls", "~> 0.8.2", require: false
+end
+
+gemspec path: "../"

gemfiles/rails_51.gemfile

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+# This file was generated by Appraisal
+
+source "https://rubygems.org"
+
+gem "actionpack", "~> 5.1"
+gem "railties", "~> 5.1"
+
+group :development do
+  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", "~> 0.51.0"
+  gem "coveralls", "~> 0.8.2", require: false
+end
+
+gemspec path: "../"