Fix dependency order between DataShiftException and DataShift::Logging

```bundle exec thor list datashift
WARNING: unable to load thorfile "...purelights.thor": uninitialized constant DataShift::Logging
.../bundler/gems/datashift-a95fb0763310/lib/datashift/exceptions.rb:11:in '<class:DataShiftException>'
No Thor commands available```

[Fixes #21]