This avoids the need to build the psycopg2 packages and thus requires
PostgreSQL dependencies.

It's not used anywhere in the code.

Otherwise the version from the CI are used and are too old

Otherwise system eggs for Django 1.9.5 are used on CI

beat.core must be 1.6.x
beat.backend.python must be 1.5.x

This fixes the issue that the examples on the latest master are using
the 1.5 version of the python backend which is not yet released.

Relates to:
beat.examples#12

There has been API changes in the latest versions that will requires some work in the
beat.web codebase

This reverts commit 8971ca83.