Workaround for missing Content-Length information

Under some scenarios, certain webservers or proxies will miss
the `Content-Length` header that is used by the progress bar
component, causing the download task to abort.

This workaround was promosed in the mailing list and is hard to
reproduce.
Upgrade RubyGems to version 1.8.29

This one is used by Ruby 1.8.x and 1.9.x so an update to latest
available in the 1.8.x series was recommended.