vagrantで仮想マシンを起動しようとしたら、boxがなかったので、'bento/centos-6.8'をダウンロードしていると、

OpenSSL SSL_read: SSL_ERROR_SYSCALL, errno 60

というエラーが出て、正常にダウンロードできません。解決方法を教えてください。

MacBook-Air:MyCentOS aonoryousuke$ vagrant up
Bringing machine 'default' up with 'virtualbox' provider...
==> default: Box 'bento/centos-6.8' could not be found. Attempting to 
find and install...
default: Box Provider: virtualbox
default: Box Version: >= 0
==> default: Loading metadata for box 'bento/centos-6.8'
default: URL: https://vagrantcloud.com/bento/centos-6.8   
==> default: Adding box 'bento/centos-6.8' (v2.3.4) for provider: 
virtualbox
default: Downloading: https://vagrantcloud.com/bento/boxes/centos- 
6.8/versions/2.3.4/providers/virtualbox.box
An error occurred while downloading the remote file. The error
message, if any, is reproduced below. Please fix this error and try
again.

OpenSSL SSL_read: SSL_ERROR_SYSCALL, errno 60