get the executable name
function getExe() {
  /* istanbul ignore next: tested on all platform on travis */
  switch (process.platform) {
    case "darwin":
      return "mkcert-" + MKCERT_VERSION + "-darwin-amd64"
    case "linux":
      return "mkcert-" + MKCERT_VERSION + "-linux-amd64"
    case "win32":
      return "mkcert-" + MKCERT_VERSION + "-windows-amd64.exe"
    default:
      console.warn("Cannot generate the localhost certificate on your " +
        "platform. Please, consider contacting the developer if you can help.")
      process.exit(0)
  }
}