"An historic" is not only still acceptable in the USA, but it also (IMO) sounds better. The leading "H" is not a strong-sounding consonant, and is often dropped in both American and British English.
The 'h' is stressed in 'history', not in 'historic'. Try pronouncing 'historic' with a stress on the first syllable, it sounds wrong. Since it's softer in historic it's more natural (to me anyway) to use 'an'.
(Also, I'm french so I barely pronounce h's to begin with - so in my speech the stressed 'h' in history is kinda soft, and the unstressed h in historic is barely there.)