Import path for new opengever.pdfconverter changed

`opengever.pdfconverter` changes the location of a constant used by `opengever.core`.

`opengever.core` tries to import the `pdfconverter` classes and constants and if it fails it assumes `opengever.pdfconverter` is not available. There is no error message (which is intended).

This PR updates will restore the functionality for the `master` of `opengever.pdfconverter`, but will break the compatibility to all previous releases.

https://github.com/4teamwork/opengever.core/blob/master/opengever/document/browser/overview.py#L15

https://github.com/4teamwork/opengever.pdfconverter/pull/3