fixing error with load_config

@alexcoco @kmcphillips 

Right now there's a crashing error with the gem because `Configuration` attempts to use `load_config` as an instance method, but it's defined as a class method.

The tests didn't find this error because the were stubbing the `load_config` method.

These changes won't 100% prevent this type of error in the future, but I think they're a step in the right direction.

I ended up making the `load_config` method public, because I don't like stubbing private methods. I'm also open to using dependency injection -- let me know what you'd prefer.

I'd like to get these fixes in soon because the broken state of the gem is blocking my other project.