Make rspec and rdf load runtime dependencies

RSpec, RSpec Its, and RDF.rb are required to run this code. When marked
as development dependencies, they are not included in the dependency
tree for external projects that use this code.

Author: Tom Johnson

rdf-spec.gemspec

@@ -27,9 +27,9 @@ Gem::Specification.new do |gem|
 
   gem.required_ruby_version      = '>= 1.9.2'
   gem.requirements               = []
-  gem.add_development_dependency 'rdf',       '~> 1.1'
-  gem.add_development_dependency 'rspec',     '~> 3.0'
-  gem.add_development_dependency 'rspec-its', '~> 1.0'
+  gem.add_runtime_dependency     'rdf',       '~> 1.1'
+  gem.add_runtime_dependency     'rspec',     '~> 3.0'
+  gem.add_runtime_dependency     'rspec-its', '~> 1.0'
   gem.add_development_dependency 'yard' ,     '~> 0.8'
   gem.post_install_message       = nil
 end