Fixes

• Default the --feed-url flag for porter mixins install to DeisLabs (#582)
• Fix installation of packr2 (#589)
• Fix bundle validation by fetching/supplying definitions schema (#581)
• Update to latest of cnab-go PR 112 (#580)

Install or Upgrade

Run (or re-run) the installation from https://porter.sh/install to get the latest version of porter.