added yard to gemspec + reordered client initialization

Author: amite

contentstack.gemspec

@@ -40,4 +40,5 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'awesome_print'
   spec.add_development_dependency 'webmock', '~> 2.3', '>= 2.3.1'
   spec.add_development_dependency 'dotenv', '~> 2.1', '>= 2.1.1'
+  spec.add_development_dependency 'yard'
 end