Be more specific about the lookup of an Ollie::Base subclass.

On [line 27 of checker.rb](https://github.com/modcloth-labs/ollie/blob/master/lib/ollie/checker.rb#L27), Ollie does a lookup of the class within the Ollie namespace to instantiate the checker for the key given. It fails in cases where the new class isn't loaded quickly enough to be chosen by the call to `.const_get`.

To illustrate this, consider a Rails app using Redis. We're interested in fetching some info, so we write an `Ollie::Redis` class which 

The `Redis` class is defined on the main `Object` namespace. If the lookup occurs before the `Ollie::Redis` class I've defined is loaded, then I Ollie tries to instantiate a new `Redis` object, because `.const_get` is called before `Ollie::Redis` is loaded.

This fix forces the key to be loaded from the `Ollie` namespace as the README prescribes.