Changed text width scale factor for metapost

I noticed that in the rendered PDF slides, the auto-wrapped texts usually do not fill the entire slide width. Here are the examples before and after I changed the font scaling factor.

![bad](https://cloud.githubusercontent.com/assets/5135317/3465884/7c977084-0273-11e4-87ba-e0e94a6e4798.png)
![good](https://cloud.githubusercontent.com/assets/5135317/3465885/7fd81b2c-0273-11e4-8e58-fa89ba51dd21.png)

 I do not know if this is font-dependent, but I noticed the same effect on [Jonathan's slides](http://cs.stanford.edu/~pliang/papers/freebase-emnlp2013-talk.pdf) (PDF pages 5, 14, 24) as well.