Show the version selector at smaller screen widths

The dropdown doesn’t show up on “mobile”:

<img width="467" alt="nay" src="https://user-images.githubusercontent.com/10070176/40442213-d782a608-5e77-11e8-8fa7-836aefe8caba.png">

It does at “desktop” widths:
<img width="252" alt="yay" src="https://user-images.githubusercontent.com/10070176/40442242-e7c4008e-5e77-11e8-9a6e-d1440856c9cb.png">
