Allow Flog enabled platforms to be configurable.

Minor change to allow configuration of platforms under which flog is enabled.  Moves currently hard-coded `mri-1.9.3` option into a default in `config.rb` and changes `flog.rake` to read the setting.  Projects wishing to configure this would add, e.g:

    # config/flog.yml
    enabled_platforms: [mri-2.0.0, mri-1.9.3]

I'm not sure if *enabled_platforms* is the correct term to use since `flog.rake` currently uses *compatible_scores*.  The former seems more intuitive to me but also implies a wider scope.  Happy to change the terms as you see fit.