==> Provider 'virtualbox' not found. We'll automatically install it now... The installation process will start below. Human interaction may be required at some points. If you're uncomfortable with automatically installing this provider, you can safely Ctrl-C this process and install it manually. ==> Downloading VirtualBox 5.0.10... This may not be the latest version of VirtualBox, but it is a version that is known to work well. Over time, we'll update the version that is installed. ==> Installing VirtualBox. This will take a few minutes... You may be asked for your administrator password during this time. If you're uncomfortable entering your password here, please install VirtualBox manually.
VirtualBox 5.0.10 がインストールされてしまうので、vagrantを1.8.5以上にします。
brew cask install vagrant