Fetch feedzirra gem directly from Github instead of rubygems.

This solves a problem with the dependency version of Builder gem (Rails and Feedzirra from rubygems depend on incompatible versions of Builder). Not a solution I'm very happy with. Maybe it's time to look for other gems to work with feeds.
Removed feedzirra from gemfile.

I don't like the fact that I have to fetch this gem from github to solve incompatibilities with Rails.

I will try to implement functionality based on the RSS parser bundled with Ruby, only if it's not enough I will study RSS gems again.