Could not find mixlib-shellout-1.6.0 in any of the sources

When I do a clean checkout and try to bundle, I get the error:
Could not find mixlib-shellout-1.6.0 in any of the sources

Maybe you could update the gem versions?
Changing the gemfile to

```
source 'http://rubygems.org'

gem 'knife-solo'
gem 'librarian-chef'
gem 'vagrant-vbguest'
gem 'vagrant-omnibus'
```

Then bundle update, seems to work.
