fixup Rakefile

Author: toshimaru

Diff for: Rakefile

@@ -23,7 +23,7 @@ end
 desc 'Generate and build documentation for older versions of Rails'
 task :build_multi, [:versions] do |_t, args|
   rails_versions = config['rails_versions'].reverse_each.to_h # Versions from oldest to newest
-  if args[:versions].is_a?(String)
+  unless args[:versions].nil?
     versions = args[:versions].split(',')
     rails_versions.select! { |version, _| versions.include?(version) }
   end