Fix margin on .media-primary--showcase

On mobile, articles with `.media-primary--showcase` for main image have an additional 10px margin along the right-hand side. This fixes that.

Bug:
![screen shot 2014-11-07 at 12 14 59](https://cloud.githubusercontent.com/assets/813383/4952707/e406093e-6677-11e4-8945-c5014462f64f.png)

/cc @sndrs