I would not expect them to update the 15-85mm any time soon.
The 18-135mm was updated mainly to include STM, since it is sold as a kit lens with the T4i/650D. The T4i/650D is Canon's first dSLR to provide AF during video shooting, and STM lenses are optimized for that use (smooth AF, as opposed to the 'jerky' AF you get with micromotor or USM lenses).
The 15-85mm lens is just about the same price as the T4i itself, and lenses at that price point are not appealing to most Rebel/xxxD shooters. So, likely no need for STM in that lens. Maybe if Canon adds video AF to a higher end body, we'll see more STM lenses down the line...maybe. But there's a trade-off - STM lenses focus more slowly than their USM counterparts, and that's going to be a turnoff for many users of high end lenses. The 18-135mm was not USM anyway, but micromotor - STM made is smoother without a focus speed penalty.
Optically, the 15-85mm is already delivering L-series performance (though without the L-series build). Given that, I'm not sure there's much of a need for an update.