无法安装delve（golang调试器）：“找不到软件包“ gopkg.in/yaml.v2”

When installing delve in OSX:

brew install go-delve/delve/delve

I get:

cannot find package "gopkg.in/yaml.v2" in any of:
    /usr/local/Cellar/go/1.4.2/libexec/src/gopkg.in/yaml.v2 (from $GOROOT)
    /private/tmp/delve20160602-37993-zqtryr/delve-0.11.0-    alpha/src/gopkg.in/yaml.v2 (from $GOPATH)

Therefore I install gopkg.in/yaml.v2 with:

go get gopkg.in/yaml.v2

Got no error. Then tried again, but problem persists.

I wonder if it has something to do with GOPATH, it is set in:

/Users/macbox/Desktop/golang

How can I see the installed packages?