Update capybara requirement from ~> 2.6.2 to ~> 3.39.2

Updates the requirements on [capybara](https://github.com/teamcapybara/capybara) to permit the latest version.
- [Changelog](https://github.com/teamcapybara/capybara/blob/master/History.md)
- [Commits](teamcapybara/capybara@2.6.2...3.39.2)

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

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

rspec_api_documentation.gemspec

@@ -22,7 +22,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency "fakefs", "~> 0.6.0"
   s.add_development_dependency "sinatra", "~> 1.4.7"
   s.add_development_dependency "aruba", "~> 0.14.14"
-  s.add_development_dependency "capybara", "~> 2.6.2"
+  s.add_development_dependency "capybara", "~> 3.39.2"
   s.add_development_dependency "rake", "~> 10.5.0"
   s.add_development_dependency "rack-test", "~> 0.6.3"
   s.add_development_dependency "rack-oauth2", "~> 1.12.0"