install ordering -- need to ensure brewcask provider functioning before packages

[mroth]

on a boxen run on a fresh machine, all the brew cask package installs fail with:

Failed running sudo -E -u username /opt/boxen/homebrew/bin/brew cask install pkgname
Error: Unknown command: cask

I suspect what's going on here is we need to make sure the brew cask provider is installed prior to any brewcask packages being evaluated and/or refresh the environment somehow after installing it so that it's recognized.