Well, for any person that happens to be in a jam. I had put a 13mm in there initially when it happened, but it snapped the first socket I put on there and stripped the second one(lesson: $10 socket sets at autozone are really junk). So I wasn't I read some non descript comments all over the web about taking it off. Some mentioned a 15mm and some mentioned a star/torx screw. I am guessing that people were talking about different engine sizes, but didn't say so.
The bolt to take off on the tensioner assembly is a 13mm. It can be pretty hard to get out of there, so be ready to apply some force. Also it is a normal screw, not left-handed or reverse threaded.