Allow specs to be run in random order
Update changelog
Remove object extension on inserted/retrieved objs

`Module#extend` invalidates the global method cache which can
drastically reduce performance for all method calls in the Ruby process.
Adjust rebased code not to use `.id`
Merge branch 'remove-persisted-object-extension'

This stops mappers from extending Perpetuity::PersistedObject onto
objects when persisting/retrieving them. Extending objects busts method
caches and should be avoided whenever you care about performance.
Update changelog
Bump version to 0.6.0