migrate to go-modules

Signed-off-by: Valentin Rothberg <rothberg@redhat.com>

vendor/github.com/mattn/go-shellwords/util_go15.go

@@ -0,0 +1,24 @@
+// +build !go1.6
+
+package shellwords
+
+import (
+	"os"
+	"os/exec"
+	"runtime"
+	"strings"
+)
+
+func shellRun(line string) (string, error) {
+	var b []byte
+	var err error
+	if runtime.GOOS == "windows" {
+		b, err = exec.Command(os.Getenv("COMSPEC"), "/c", line).Output()
+	} else {
+		b, err = exec.Command(os.Getenv("SHELL"), "-c", line).Output()
+	}
+	if err != nil {
+		return "", err
+	}
+	return strings.TrimSpace(string(b)), nil
+}