I am trying to try out the migrate CLI, and am running into a problem with the installation steps in the README. I run these commands:

$ go get -u -d github.com/golang-migrate/migrate/cli github.com/lib/pq
$ go build -tags 'postgres' -o /usr/local/bin/migrate github.com/golang-migrate/migrate/cli
go build github.com/golang-migrate/migrate/database/postgres: no buildable Go source files in {GOPATH}/src/github.com/golang-migrate/migrate/database/postgres

If I run the go build with a tag other than postgres it works.