Fix reference to task's subtitle_version when getting the perform URL.
Fix reference to task's subtitle_version when getting the perform URL.

Somewhat fixes https://unisubs.sifterapp.com/projects/12298/issues/767418/comments

ofer43211/unisubs,ofer43211/unisubs,ofer43211/unisubs,pculture/unisubs,wevoice/wesub,eloquence/unisubs,norayr/unisubs,wevoice/wesub,eloquence/unisubs,norayr/unisubs,norayr/unisubs,eloquence/unisubs,ReachingOut/unisubs,wevoice/wesub,ofer43211/unisubs,ReachingOut/unisubs,ReachingOut/unisubs,pculture/unisubs,ReachingOut/unisubs,wevoice/wesub,norayr/unisubs,pculture/unisubs,ujdhesa/unisubs,pculture/unisubs,ujdhesa/unisubs,ujdhesa/unisubs,ujdhesa/unisubs,eloquence/unisubs