Update multi_json requirement from ~> 1.11.2 to ~> 1.15.0

Updates the requirements on [multi_json](https://github.com/intridea/multi_json) to permit the latest version.
- [Changelog](https://github.com/intridea/multi_json/blob/master/CHANGELOG.md)
- [Commits](intridea/multi_json@v1.11.2...v1.15.0)

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

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

Author: dependabot[bot]

rspec_api_documentation.gemspec

@@ -35,7 +35,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency "minitest", "~> 5.8.4"
   s.add_development_dependency "contracts", "~> 0.17"
   s.add_development_dependency "gherkin", "~> 3.2.0"
-  s.add_development_dependency "multi_json", "~> 1.11.2"
+  s.add_development_dependency "multi_json", "~> 1.15.0"
   s.add_development_dependency "rspec", "~> 3.0"
 
   s.files        = Dir.glob("lib/**/*") + Dir.glob("templates/**/*")