Update rake development dependency to 13

Author: lautis

piperator.gemspec

@@ -24,6 +24,6 @@ Gem::Specification.new do |spec|
   spec.require_paths = ['lib']
 
   spec.add_development_dependency 'bundler', '>= 1.14'
-  spec.add_development_dependency 'rake', '~> 12.0'
+  spec.add_development_dependency 'rake', '~> 13.3'
   spec.add_development_dependency 'rspec', '~> 3.0'
 end