don't allow width collapsing in completion popup

This fixes a visual 'bug' where the left cell of the popup would collapse when the window width was too small, producing a display like:

![screen shot 2014-11-22 at 4 00 27 pm](https://cloud.githubusercontent.com/assets/1976582/5156111/b86ef390-7260-11e4-9a74-c2a433b789c4.png)

We set the `min-width` property to ensure that table cell resizing rules don't override our width setting.