Fix check in spec_helper.rb to work even with --unguarded

Author: eregon

spec/ruby/spec_helper.rb

@@ -27,7 +27,8 @@ def report_on_exception=(value)
   end
 end
 
-ruby_version_is ""..."2.7" do
+# Compare with SpecVersion directly here so it works even with --unguarded
+if VersionGuard::FULL_RUBY_VERSION < SpecVersion.new('2.7')
   abort "This version of ruby/spec requires Ruby 2.7+"
 end