Fix RETableViewLongTextCell constraints on iOS 8

Before
![img_6829](https://cloud.githubusercontent.com/assets/577516/4213281/5d675a1a-38b2-11e4-8e4b-3c741754e5cf.png)

After
![screen_shot_2014-09-10_at_15_20_12](https://cloud.githubusercontent.com/assets/577516/4213288/a881ff00-38b2-11e4-9999-2ac56eb26f84.png)