Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Just a quick draft for an AP Alistar. Both Pulverize and Headbutt add damage 1:1 for AP so in the end both skills will deal a base damage close to 700. In addition his Triumphant Roar will actually be worthwhile, healing for much more at a low cost.
AP Alistar will still be somewhat tanky at the end of the game due to his large HP base. He will help his team more through damage dealt at initiation and healing than with aura items.
The items used are mainly for AP, with Rod of Ages and Rylai's Crystal Scepter for some much needed HP and Mana, and Abyssal Scepter for some much needed magic resist. Ninja Tabi are the only source of armor, but also give dodge. Can be replaced by mercury treads if desired.
A Banshee's Veil may be a suitable replacement to help initiate; however, Unbreakable Will can get rid of some crowd control.
I haven't tried this build yet, but I'll post if it's actually viable.