fix bug 1115738 - smaller width when ip is shown

Use 18% width for revision dashboard columns when showing the (5th) IP column.

I hard-coded the 18% value, which is probably wrong. ( @darkwing, @openjck, @stephaniehobson ? )

I also consolidated some stylus selectors, which feels less wrong but still might be wrong?

Anyway, it looks right:

![mdn-rev-dash-columns](https://cloud.githubusercontent.com/assets/71928/5559339/cc50dfe0-8d0e-11e4-8de0-a6bf2dfee617.png)