fix - require 'mini_gmagick'

Fix for cases where `require 'mini_gmagick'`.
I understand you've made some changes to setting #processor, and setting it directly is pretty much deprecated (I think).

There may be a more correct way of doing this/you may just want to drop this file or issue a deprecation warning or whatever.

I agree that using a configure block makes more sense, as below: 
```ruby
MiniMagick.configure do |config|
  config.cli = :graphicsmagick
end
```

Anyway, thanks for the gem!