Shawn Ashmore, who dated Michelle Trachtenberg from 2004 to 2006, paid tribute to the Buffy the Vampire Slayer star after she died Feb. 26 at the age of 39.  Shawn Ashmore is honoring his ex. The X-Men alum, who dated Michelle Trachtenberg from 2004 to 2006, shared a tribute to the late actress following news…