message: Fix hash slice comparison for Rust version < 1.49
  Prior to 1.49 there's no comparison between Vec<u8> and [u8]. Use
  a slice instead.
diff: @@ -253,7 +253,7 @@ impl Npk {
  Error::Manifest(format!("Failed to parse manifest hash {}", e))
  })?;
  let actual_hash = Sha256::digest(&content.as_bytes());
  - if expected_hash != actual_hash[..] {
  + if expected_hash != actual_hash.as_slice() {
  return Err(Error::Manifest(format!(
  "Invalid manifest hash (expected={} actual={})",
  manifest_hash,
  