Update dependencies and run Ruby 3.0 on CI.

Author: gkellogg

.github/workflows/ci.yml

@@ -23,6 +23,7 @@ jobs:
           - 2.5
           - 2.6
           - 2.7
+          - 3.0
           - ruby-head
           - jruby
     steps:

rdf-turtle.gemspec

@@ -20,7 +20,7 @@ Gem::Specification.new do |gem|
 
   gem.required_ruby_version = '>= 2.4'
   gem.requirements          = []
-  gem.add_runtime_dependency     'rdf',             '~> 3.1', '>= 3.1.2'
+  gem.add_runtime_dependency     'rdf',             '~> 3.1', '>= 3.1.8'
   gem.add_runtime_dependency     'ebnf',            '~> 2.1'
   gem.add_development_dependency 'erubis',          '~> 2.7'
   gem.add_development_dependency 'htmlentities',    '~> 4.3'