Add min Ruby version (2.7) to gemspec

Author: fbacall

ro_crate.gemspec

@@ -8,6 +8,7 @@ Gem::Specification.new do |s|
   s.homepage    = 'https://github.com/ResearchObject/ro-crate-ruby'
   s.require_paths = ['lib']
   s.licenses    = ['MIT']
+  s.required_ruby_version = '>= 2.7.0'
   s.add_runtime_dependency 'addressable', '>= 2.7', '< 3'
   s.add_runtime_dependency 'rubyzip', '>= 2.3', '< 4'
   s.add_development_dependency 'rake', '~> 13.4.2'