Sync more gem versions.

Author: gkellogg

rdf-virtuoso.gemspec

@@ -15,12 +15,14 @@ Gem::Specification.new do |s|
   s.files         = %w(README.md LICENSE VERSION) + Dir.glob('lib/**/*.rb')
   s.require_paths = ['lib']
 
-  s.add_runtime_dependency 'rdf', '~> 3.1'
-  s.add_runtime_dependency 'httparty', '~> 0.20'
-  s.add_runtime_dependency 'api_smith', '~> 1.3.0'
+  s.required_ruby_version      = '>= 2.6'
 
-  s.add_development_dependency 'rspec', '~> 3.10'
-  s.add_development_dependency 'rdf-spec', '~> 3.2'
+  s.add_runtime_dependency 'rdf',           '~> 3.2'
+  s.add_runtime_dependency 'httparty',      '~> 0.20'
+  s.add_runtime_dependency 'api_smith',     '~> 1.3.0'
+
+  s.add_development_dependency 'rspec',     '~> 3.10'
+  s.add_development_dependency 'rdf-spec',  '~> 3.2'
   s.add_development_dependency 'rdf-vocab', '~> 3.2'
 
 end