Make rexml a runtime dependency.

Author: gkellogg

rdf-spec.gemspec

@@ -25,7 +25,7 @@ Gem::Specification.new do |gem|
   gem.add_runtime_dependency     'rdf-isomorphic',  '~> 3.1'
   gem.add_runtime_dependency     'rspec',           '~> 3.9'
   gem.add_runtime_dependency     'rspec-its',       '~> 1.3'
-  gem.add_development_dependency 'rexml',           '~> 3.2'  # For Webmock
+  gem.add_runtime_dependency     'rexml',           '~> 3.2'  # For Webmock
   gem.add_runtime_dependency     'webmock',         '~> 3.7'
   gem.add_development_dependency 'yard' ,           '~> 0.9.20'
   gem.post_install_message        = nil