Special case safe_architecture for Raspbian and Pidora

Fixes #352
I think this might be a little easier to follow:

```ruby
when 'armv6l'
  # INSERT BIG EXPLANATION COMMENT HERE
  'armhf'
else
  Ohai['kernel']['machine']
end
```

Basically we will always want to map the kernel architecture of `armv6l` to the Debian architecture of `armhf`.