Fixes ranking of firmware versions to match semver.

This now lists 0.1.11 at top:

```
➜  cli git:(master) ✗ tessel update --list
INFO Switch to any of these builds with `tessel update -b <build name>`
  o current
  o 0.1.11
  o 0.1.10
  o 0.1.9
  o 0.1.8
  o 0.1.7
  o 0.1.6
  o 0.1.5
  o 0.1.4
  o 0.1.3
  ...
```

The old sorting code was checking a property, `.url`, that didn't exist anyway.