Center definition play button icon

@sdougbrown @chrismorast 

Before:
![screen shot 2014-06-04 at 12 10 04 pm](https://cloud.githubusercontent.com/assets/126358/3176205/bdc42ab6-ec02-11e3-80df-845b30af50ba.png)

After:
![screen shot 2014-06-04 at 12 08 19 pm](https://cloud.githubusercontent.com/assets/126358/3176204/bc0455c0-ec02-11e3-82ba-978add0f9210.png)