Update gemfile git call

Calling bundle install using Ruby 2.0 and Git 1.8.4 was causing the following error for me:

```bash
$ bundle install
Fetching gem metadata from https://rubygems.org/.......
Fetching gem metadata from https://rubygems.org/..
Fetching git://github.com/alindeman/kitabu.git
fatal: unable to connect to github.com:
github.com[0: 192.30.252.130]: errno=Operation timed out

Git error: command `git clone 'git://github.com/alindeman/kitabu.git'
"/.gem/ruby/2.0.0/cache/bundler/git/kitabu-4ca678e59725889bd981d7d445b1937f5598e99b"
--bare --no-hardlinks` in
directory /upgrading_to_rails_4 has failed.
```

This commit updates the Gemfile to fall more in line with what is described in http://bundler.io/v1.3/git.html