fix: pin simplecov version for Ruby 2.4 compatibility

Author: Sam Harrison

ruby_http_client.gemspec

@@ -19,5 +19,5 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'minitest'
   spec.add_development_dependency 'rake'
   spec.add_development_dependency 'rubocop', '~> 0.88.0'
-  spec.add_development_dependency 'simplecov'
+  spec.add_development_dependency 'simplecov', '~> 0.18.5'
 end