Bugfix: properties visibility changed

Given there is a class `EntityImage` which extends `Image`. And `Image` uses the following traits: `Timestampable` + `Sortable`. When generating the database based on `EntityImage` annotations, doctrine will only create the `Timestampable` fields.

Setting the trait properties to `protected` fixes this issue and creates the db fields correctly as doctrine annotation reader now can see all the `EntityImage` properties.

Build passed:
https://travis-ci.org/illarra/DoctrineBehaviors/builds/9986692