Fix formatting consistency in app.js

Following up on my prior pull request where I missed a similar formatting issue at line 23. 

Please disregard my prior pull request, currently #9

I noticed that quotation marks weren't consistent and I thought it would look better if the quotes were consistent throughout the file.

-Before
![screen shot 2014-07-11 at 10 31 10 am](https://cloud.githubusercontent.com/assets/5373129/3558497/73149d64-093c-11e4-8313-0be2a31c591d.png)


-After

![screen shot 2014-07-11 at 1 43 26 pm](https://cloud.githubusercontent.com/assets/5373129/3558471/60c40bae-093c-11e4-9beb-d72135a6b9b2.png)