Add provisioning steps for older boxes

[byrond]

• Adds a step to update the expired Yarn apt key.
• Adds a step to remove the "duplicity-team" PPA repository that no longer exists.

Testing

• Set the version constraints for a project to use < 1.4.0
• vagrant destroy
• vagrant up
• Verify provisioning completes (especially the "Update apt cache" step)
• Set the version constraints to allow 1.4.0 and above.
• vagrant destroy
• vagrant up
• Verify provisioning completes as above.

Note: This was tested with box versions 1.3.0 and 1.4.1.

[becw]

This looks great, and works cleanly for me.