installation: dependency links renovation

* Prepares `setup.py` for Invenio v2.0 release for pip<1.6.
  (addresses #1797)

* NOTE use `pip install --process-dependency-links -e .`

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
https://github.com/mitsuhiko/flask-sqlalchemy/releases/tag/2.0 !!