You may be getting rate limited by Github

docker-machinアップグレードしようとしたらこんなエラーが

$ docker-machine upgrade dev
Detecting the provisioner...  
Downloading latest boot2docker iso...  
Error demarshaling the Github API response: json: cannot unmarshal object into Go value of type []struct { TagName string "json:\"tag_name\"" }  
You may be getting rate limited by Github.  

どうやら制限がかかったらしい
GitHubのトークンを指定してあげればOK

$ docker-machine --github-api-token=<トークン> upgrade dev
Detecting the provisioner...  
Downloading latest boot2docker iso...  
Latest release for github.com/boot2docker/boot2docker is v1.9.1  
Downloading https://github.com/boot2docker/boot2docker/releases/download/v1.9.1/boot2docker.iso to /Users/kohta/.docker/machine/cache/boot2docker.iso...  
0%....10%....20%....30%....40%....50%....60%....70%....80%....90%....100%  
Stopping machine to do the upgrade...  
Upgrading machine "dev"...  
Starting machine back up...  
(dev) OUT | Starting VM...

参考

"You may be getting rate limited by Github" error message