forecast.css: Change bottom margin to -.5em.

As mentioned in https://github.com/duckduckgo/zeroclickinfo-spice/pull/876#issuecomment-45955868

@chrismorast @sdougbrown @bsstoner 

![ios_iphone_5s](https://cloud.githubusercontent.com/assets/81969/3264885/ec747346-f285-11e3-829a-b2bebd57f105.jpg)