Set the timezone to GMT for display of dates in the trends.

The displayed dates were off by a day (see screenshots of the old vs. new).

![date-old](https://cloud.githubusercontent.com/assets/5265058/4292105/47657586-3dca-11e4-89d2-7bbcf8b5ac35.png)
![date-updated](https://cloud.githubusercontent.com/assets/5265058/4292106/4773b18c-3dca-11e4-9273-c9300a484387.png)

@clintonb @rocha