Detect & fix homebrew update bug from Aug 10-11.
Detect & fix homebrew update bug from Aug 10-11.

This checks if homebrew has not been updated since Aug 15 (a few days
after the bug). If so, it forces and update to fix the bug.

I tested this by resetting my homebrew to Aug 11:

```
cd /usr/local; git reset --hard b7dc6ce
```

…then running this script.

kickstarter/laptop