按照视频的步骤 vagrant up 之后出现问题(Mac OS) ：
vagrant up 之后出现：
No usable default provider could be found for your system. Vagrant relies on interactions with 3rd party systems, known as "providers", to provide Vagrant with resources to run development environments. Examples are VirtualBox, VMware, Hyper-V. The easiest solution to this message is to install VirtualBox, which is available for free on all major platforms. If you believe you already have a provider available, make sure it is properly installed and configured. You can see more details about why a particular provider isn't working by forcing usage with `vagrant up --provider=PROVIDER`, which should give you a more specific error message for that particular provider.
怎么办？ stack overflow上面说要设置环境变量http://stackoverflow.com/questions/29450437/unable-to-vagrant-up-how-to-set-providers 可还是不知道怎么做。。
