Genericize Version Checker Modules

Based on #125, this genericizes version checker modules such that they are automatically loaded from the polisher/adaptors/version_checker dir
@movitto 
It feels like we need a class hierarchy here

```ruby
class BaseVersionChecker
  def self.default?
    DEFAULT || true
  end
end

class ErrataVersionChecker < BaseVersionChecker
  DEFAULT = false
end

class FedoraVersionChecker < BaseVersionChecker
end
...
```

Just a thought.  It feels like you have other logic we can put in the base and maybe move to instantiating the version checkers instead of using class methods.