Fix reloading of additional_permitted_attributes

The additional_permitted_attributes method, which is generated by this
plugin, only worked in production or the first development-request.

Now we insert the additional permitted attributes on every code reload.
Merge pull request #6 from opf/fix/reloading_of_additional_permitted_attributes

Fix reloading of additional_permitted_attributes
Bump version to 3.0.8

Due to our (new) convention plugins inherit their versions from the
OpenProject core. The next stable release of the OpenProject core is
3.0.8.
Merge branch 'release/3.0' into dev
Bump version to corresponding core version