Update httparty to >= 0.21.0

Gemfile

@@ -1,7 +1,3 @@
 source 'https://rubygems.org'
 
-gem 'rake', '12.3.3'
-gem 'httparty', '0.16.3'
-gem 'rubocop', '0.49.0'
-
 gemspec

octocatalog-diff.gemspec

@@ -25,7 +25,7 @@ Gem::Specification.new do |s|
 EOF
 
   s.add_runtime_dependency 'diffy', '>= 3.1.0'
-  s.add_runtime_dependency 'httparty', '>= 0.11.0'
+  s.add_runtime_dependency 'httparty', '>= 0.21.0'
   s.add_runtime_dependency 'hashdiff', '>= 0.3.0'
   s.add_runtime_dependency 'parallel', '>= 1.12.0'
   s.add_runtime_dependency 'rugged', '>= 0.25.0b2'