Efficient latex updates mess with minus signs

[mikehaverstock]

I was seeing issues with evaluations in the graphing calculator that we sometimes use a real minus sign and sometimes use a special unicode one. We should investigate what's going on and make sure that efficient latex updates use the correct minus sign. I think the issue was that mathquill started off with the special minus sign. Then the slider went positive. This caused the minus sign to disappear. When the number went back negative we used a different minus sign in the efficient update logic than mathquill originally inserted.