Use template literals

[alfredodeza]

To prevent problems like this one:

  } else {
    prefix = "The container image contains software with a vulnerability: ";
  }
  return prefix + "(" + v.package + " type=" + v.package_type + ")";

See: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals

[kzantow]

This is superseded by #105