I just achieved rank 20 in Hearthstone can I still get the card back even though its 31 of july

You have to complete all 5 wings on heroic mode to unlock the new card back.

Source

Completing Naxxramas on Heroic mode (Quest: The Fall of Naxxramas)

Official word can be found here, and two posts below that:

Golden Gelbin Mekkatorque is acquired by making a purchase with the Hearthstone in-game store. It is currently still available in Open Beta. It is not craftable.

Regular Gelbin Mekkatorque can be crafted using Arcane Dust.

Golden Elite Tauren Chieftain was a part of our BlizzCon In-Game Goodies card, which was acquired by attending BlizzCon 2013 or by purchasing the BlizzCon Virtual Ticket. It is not craftable.

Regular Elite Tauren Chieftain can be crafted using Arcane Dust.

That post dates from 1/21/2014, but as far as I'm aware, there have been no changes since (except for the beta ending, so Golden Mekkatorque is no longer available).

Additionally, neither one can be drafted in the arena, according to a dev on twitter.

_{Thanks to this and this for links}