Update rspec requirement from ~> 3.4.0 to ~> 3.9.0

Updates the requirements on [rspec](https://github.com/rspec/rspec) to permit the latest version.
- [Release notes](https://github.com/rspec/rspec/releases)
- [Commits](rspec/rspec@v3.4.0...v3.9.0)

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

Author: dependabot-preview[bot]

bing-ads-reporting.gemspec

@@ -23,7 +23,7 @@ Gem::Specification.new do |gem|
   gem.add_dependency 'savon',   '~> 2.12.0'
 
   gem.add_development_dependency 'reek', '~> 5.4.0'
-  gem.add_development_dependency 'rspec', '~> 3.4.0'
+  gem.add_development_dependency 'rspec', '~> 3.9.0'
   gem.add_development_dependency 'rubocop', '~> 0.70.0'
   gem.add_development_dependency 'rubocop-performance', '~> 1.3.0'
   gem.add_development_dependency 'rubocop-rspec', '~> 1.33.0'