Bump rubocop to allow version 1.13.0

Version 1.4 of rubocop introduced a breaking change that creates some
warnings and issues when using `parser/ruby27`, this issue was fixed in
rubocop/rubocop#9702 and released in 1.13.0

Author: Tonkpils

rubocop-github.gemspec

@@ -10,7 +10,7 @@ Gem::Specification.new do |s|
 
   s.files = Dir["README.md", "STYLEGUIDE.md", "LICENSE", "config/*.yml", "lib/**/*.rb", "guides/*.md"]
 
-  s.add_dependency "rubocop", "<= 1.6.1"
+  s.add_dependency "rubocop", "<= 1.13.0"
   s.add_dependency "rubocop-performance", "<= 1.7.1"
   s.add_dependency "rubocop-rails", "<= 2.7.1"