Update dependencies.

Author: gkellogg

README.md

@@ -333,7 +333,7 @@ from BNode identity (i.e., they each entail the other)
 
 ## Dependencies
 
-* [Ruby](http://ruby-lang.org/) (>= 2.0)
+* [Ruby](http://ruby-lang.org/) (>= 2.2)
 * [LinkHeader][] (>= 0.0.8)
 * Soft dependency on [RestClient][] (>= 1.7)
 

rdf.gemspec

@@ -28,14 +28,10 @@ Gem::Specification.new do |gem|
   gem.requirements               = []
   gem.add_runtime_dependency     'link_header', '~> 0.0', '>= 0.0.8'
   gem.add_runtime_dependency     'hamster',     '~> 3.0'
-  #gem.add_development_dependency 'rdf-spec',    '~> 3.0'
-  #gem.add_development_dependency 'rdf-turtle',  '~> 3.0'
-  #gem.add_development_dependency 'rdf-vocab',   '~> 3.0'
-  #gem.add_development_dependency 'rdf-xsd',     '~> 3.0'
-  gem.add_development_dependency 'rdf-spec',    '>= 2.2', '< 4.0'
-  gem.add_development_dependency 'rdf-turtle',  '>= 2.0', '< 4.0'
-  gem.add_development_dependency 'rdf-vocab',   '>= 2.0', '< 4.0'
-  gem.add_development_dependency 'rdf-xsd',     '>= 2.0', '< 4.0'
+  gem.add_development_dependency 'rdf-spec',    '~> 3.0'
+  gem.add_development_dependency 'rdf-turtle',  '~> 3.0'
+  gem.add_development_dependency 'rdf-vocab',   '~> 3.0'
+  gem.add_development_dependency 'rdf-xsd',     '~> 3.0'
   gem.add_development_dependency 'rest-client', '~> 2.0'
   gem.add_development_dependency 'rspec',       '~> 3.7'
   gem.add_development_dependency 'rspec-its',   '~> 1.2'