Packr installation instructions no longer work

[digdilem]

Installation guide appears to be out of date.

go get github.com/gobuffalo/packr

cannot find package "github.com/gobuffalo/packr/packr" in any of:
        /usr/lib/go-1.15/src/github.com/gobuffalo/packr/packr (from $GOROOT)
        /root/go/src/github.com/gobuffalo/packr/packr (from $GOPATH)
# golang.org/x/sys/unix
/root/go/src/golang.org/x/sys/unix/syscall.go:83:16: undefined: unsafe.Slice
/root/go/src/golang.org/x/sys/unix/syscall_linux.go:2256:9: undefined: unsafe.Slice
/root/go/src/golang.org/x/sys/unix/syscall_unix.go:118:7: undefined: unsafe.Slice
/root/go/src/golang.org/x/sys/unix/sysvshm_unix.go:33:7: undefined: unsafe.Slice