Don't panic if your HEAD is detached

Examples of situations when HEAD is not symbolic:

 * You're doing interactive rebase
 * You're using git bisect
 * You love working with your head detached :smile: 
 * ... and so on.

Before this fix, when you run `boxen`, you get a crash like this:

```
fatal: ref HEAD is not a symbolic ref
/opt/boxen/bin/boxen:42: undefined method `join' for nil:NilClass (NoMethodError)
```

After the fix, you merely get a warning:

```
fatal: ref HEAD is not a symbolic ref
Boxen on a non-master branch '549f113', won't auto-update!
```

What say y'all?