Update rdf dependency.

Author: gkellogg

json-ld.gemspec

@@ -27,7 +27,7 @@ Gem::Specification.new do |gem|
 
   gem.required_ruby_version = '>= 2.2.2'
   gem.requirements          = []
-  gem.add_runtime_dependency     'rdf',             '~> 2.2'
+  gem.add_runtime_dependency     'rdf',             '~> 2.2', '>= 2.2.8'
   gem.add_runtime_dependency     'multi_json',      '~> 1.12'
   gem.add_development_dependency 'linkeddata',      '~> 2.2'
   gem.add_development_dependency 'jsonlint',        '~> 0.2'  unless RUBY_ENGINE == "jruby"