constraint why not download directly!

[wittwitt]

`
[[constraint]]

name = "golang.org/x/text"

branch = "master"

source = "github.com/golang/text"
`

why not directly download from [github.com/golang/text]
send ( http://golang.org/x/text?go-get=1 ) to get meta data ,should on the back!

Get meta data need more operations,you have to build a site

[sdboyer]

please see #860.