Add webrick to development dependencies (#1301)

`ri`'s server mode requires webrick to be installed. Since webrick has
become a bundled gem a while ago, it's required to have it in the
development dependencies explicitly.

Author: st0012

Gemfile

@@ -10,3 +10,4 @@ gem 'test-unit-ruby-core'
 gem 'rubocop', '>= 1.31.0'
 gem 'gettext'
 gem 'prism', '>= 0.30.0'
+gem 'webrick'