make the changelog table responsive to prevent overflow

Small fix:

###### Before

![screen shot 2014-11-13 at 11 55 28 am](https://cloud.githubusercontent.com/assets/13248/5035433/026ba5d8-6b2c-11e4-84ec-1a1dfcd7cd2b.png)

###### After (scrolls right)

![screen shot 2014-11-13 at 11 55 17 am](https://cloud.githubusercontent.com/assets/13248/5035432/026b1168-6b2c-11e4-94aa-61418355cfde.png)

@ciaranarcher @zendesk/samson