pytest-server-fixture: Update RethinkDB fixtures to support rethinkdb >= 2.4.0

[jcfr]

Problem: Using the latest version of the python package causes the tests/integration/test_rethink_server.py to fail.

Background: Since the API associated with the latest release of rethinkdb-python package released on March 2nd 2019 is backward incompatible, the implementation of the RethinkDB fixtures need to be updated.

See https://github.com/rethinkdb/rethinkdb-python/releases/tag/v2.4.0

[eeaston]

Thanks for raising - gosh I'm surprised the driver was updated since the database itself has ceased supported development a few years ago now.

[eeaston]

Fixed in #134