Implement Source#mcg_url
Implement /feed/sources.atom

Using URL for atom:id may not comply with spec as they
can be changed in the future.
But I don't know how to determine proper IRI.

Reference: http://tools.ietf.org/html/rfc4287#section-4.2.6
Remove commented-out code in Gemfile
Remove rerun from Gemfile

It seems `rerun -p "**/*.rb" 'bundle exec rackup'` works fine.
We don't have to use rerun via `bundle exec`.
Specify Ruby version by Gemfile and .ruby-version

Gemfile is for Bundler and Heroku.
.ruby-version is for rbenv.