Hide date on mobile

Because density

Before:
![screen shot 2014-10-21 at 11 22 38](https://cloud.githubusercontent.com/assets/1607666/4716773/4f3a2650-590c-11e4-97e3-65390b54cd0b.png)

After:
![screen shot 2014-10-21 at 11 22 29](https://cloud.githubusercontent.com/assets/1607666/4716772/4e15f8ee-590c-11e4-8c36-32729fc39542.png)