use dedicated html5.validator.nu

validator.nu seems to be offline/overloaded often,
the dedicated html5 subdomain is on a different IP
and has either more bandwidth/power or less users.
change to validator.w3.org/nu

html5.validator.nu had problems as seen in MBS-6041
Merge remote-tracking branch 'jonnyjd/validator' into beta

* jonnyjd/validator:
  change to validator.w3.org/nu
  use dedicated html5.validator.nu