Update the missing plugin page to stop taunting Safari 5.1 users, and
suggest they try out OmniWeb.

Review at http://gwt-code-reviews.appspot.com/1536803

Review by: knorton@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10631 8db76d5a-ed1c-0410-87a9-c151d255dfc7