Update styling of /builds.

Probably could still use more work, but this fixes some issues pointed out by @wycats in #emberjs-dev on IRC.

Tagged:
![/tagged](https://www.monosnap.com/image/VDjkkFyjblMnlXIKTS0IyemvC.png)

Release:
![/release](https://www.monosnap.com/image/4z9Gy4sBBfr8MT3SsUI2OYObp.png)

Beta:
![/beta](https://www.monosnap.com/image/rBBPdlJTQp9ErUhvx4LxSwt7n.png)

Canary:
![/canary](https://www.monosnap.com/image/4I9Sb5Pgkm1zcUocudFbQN3Jq.png)