Update rethinkdb dependency

The latest `warcprox` 2.5.1 requirement
https://github.com/internetarchive/doublethink/blob/Py311/setup.py
requires `rethinkdb>=2.4.9,<2.5` but Brozzler has `rethinkdb>=2.3,<2.4`
and this creates a conflict if they are in the same virtualenv.

We update Brozzler to use the same dependency.

setup.py

@@ -72,7 +72,7 @@ setuptools.setup(
             'pillow>=5.2.0',
             'urlcanon>=0.1.dev23',
             'doublethink>=0.2.0',
-            'rethinkdb>=2.3,<2.4',
+            'rethinkdb>=2.4.9,<2.5',
             'cerberus>=1.0.1',
             'jinja2>=2.10',
             'cryptography>=2.3',