InTheaters: Remove _tmb from the end of image posters.

Rotten tomatoes are returning images such as http://content6.flixster.com/movie/11/17/84/11178428_tmb.jpg and it should end in _det.
Update in_theaters.js
Merge pull request #903 from duckduckgo/jag/in_theaters-image

InTheaters: Remove _tmb from the end of image posters.