upgrade rspec

Author: mojavelinux

asciidoctor-tabs.gemspec

@@ -35,5 +35,5 @@ Gem::Specification.new do |s|
   s.add_runtime_dependency 'asciidoctor', ['>= 2.0.0', '< 3.0.0']
 
   s.add_development_dependency 'rake', '~> 13.0.0'
-  s.add_development_dependency 'rspec', '~> 3.12.0'
+  s.add_development_dependency 'rspec', '~> 3.13.0'
 end