get larger twitter user avatars using a trick from Bridgy source 

Hi Ryan, I'm sure you've considered this, but I was curious what you think about removing _normal from the Twitter user image URL in user_to_actor to get a larger profile image (I got the trick from Bridgy's twitter module)?

Feel 100% free to reject this PR, it won't hurt my feelings. I just figured it was friendlier than asking for a feature outright ;)