Fix several rubocop warnings

Metrics/LineLength: Line is too long.
RSpec/InstanceVariable: Use let instead of an instance variable.
Layout/TrailingBlankLines: Final newline missing.
Style/StringLiterals: Prefer double-quoted strings.

config/environments/development.rb

@@ -13,7 +13,7 @@ Rails.application.configure do
   config.consider_all_requests_local = true
 
   # Enable/disable caching. By default caching is disabled.
-  if Rails.root.join('tmp/caching-dev.txt').exist?
+  if Rails.root.join("tmp/caching-dev.txt").exist?
     config.action_controller.perform_caching = true
 
     config.cache_store = :memory_store