fix travis

somehow the dev.go file was ignored by Git

vendor/github.com/mvdan/sh/interp/dev.go

@@ -0,0 +1,21 @@
+package interp
+
+import (
+	"io"
+)
+
+var _ io.ReadWriteCloser = devNull{}
+
+type devNull struct{}
+
+func (devNull) Read(_ []byte) (int, error) {
+	return 0, io.EOF
+}
+
+func (devNull) Write(p []byte) (int, error) {
+	return len(p), nil
+}
+
+func (devNull) Close() error {
+	return nil
+}