Log Message:
Update ruby-gettext to 1.91.0

Notes:
 * Works again on Win32 + JRuby-1.1.1
 * Fixed memory leaks.
 * Fixed bugs.
 * Speed up.
 * Support Serbian(sr)
 * Rails Support
    * rubyonrails-1.2.x isn't supported(2.0.0 or later are supported).
    * Localize ActionView::Helpers::FormBuilder#label.
      e.g.)
       <% form_for(@article) do |f| %>
           <p><%= f.label :lastupdate %></p>
       <% end %>
        => msgid "Article|Lastupdate" is used as the label string.