Update rspec requirement from ~> 3.12.0 to ~> 3.13.0

Updates the requirements on [rspec](https://github.com/rspec/rspec-metagem) to permit the latest version.
- [Commits](rspec/rspec-metagem@v3.12.0...v3.13.0)

---
updated-dependencies:
- dependency-name: rspec
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

netsuite.gemspec

@@ -22,6 +22,6 @@ Gem::Specification.new do |gem|
 
   gem.add_dependency 'savon', '>= 2.3.0', '!= 2.13.0'
 
-  gem.add_development_dependency 'rspec', '~> 3.12.0'
+  gem.add_development_dependency 'rspec', '~> 3.13.0'
   gem.add_development_dependency 'rake', '~> 12.3.3'
 end