Update rspec requirement from ~> 3.11.0 to ~> 3.12.0 (#567)

Updates the requirements on [rspec](https://github.com/rspec/rspec-metagem) to permit the latest version.
- [Release notes](https://github.com/rspec/rspec-metagem/releases)
- [Commits](rspec/rspec-metagem@v3.11.0...v3.12.0)

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

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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', '~> 3.11.0'
+  gem.add_development_dependency 'rspec', '~> 3.12.0'
   gem.add_development_dependency 'rake', '~> 12.3.3'
 end