Limit version of cassandra-driver

[cdonati]

This avoids using version 3.18.0, which does not seem to be able to connect to the cluster properly.

[cdonati]

Demo
prime_db.log after bash bootstrap.sh:

2019-08-07 22:51:44,698 WARNING schema.py:282 [control connection] Error connecting to 10.10.5.119:9042: 
Traceback (most recent call last):
  File "cassandra/cluster.py", line 2928, in cassandra.cluster.ControlConnection._reconnect_internal
    return self._try_connect(host)
  File "cassandra/cluster.py", line 2985, in cassandra.cluster.ControlConnection._try_connect
    raise
  File "cassandra/cluster.py", line 2982, in cassandra.cluster.ControlConnection._try_connect
    self._refresh_schema(connection, preloaded_results=shared_results, schema_agreement_wait=-1)
  File "cassandra/cluster.py", line 3084, in cassandra.cluster.ControlConnection._refresh_schema
    self._cluster.metadata.refresh(connection, self._timeout, **kwargs)
  File "cassandra/metadata.py", line 127, in cassandra.metadata.Metadata.refresh
    parser = get_schema_parser(connection, server_version, timeout)
  File "cassandra/metadata.py", line 2766, in cassandra.metadata.get_schema_parser
    version = Version(server_version)
  File "cassandra/util.py", line 1254, in cassandra.util.Version.__init__
    self.patch = int(parts.pop()) if parts else 0
ValueError: invalid literal for int() with base 10: '2-SNAPSHOT'

prime_db.log after bash bootstrap.sh --repo http://github.com/cdonati/appscale --branch cassandra-driver-version-3.7:

2019-08-07 23:00:58,709 INFO schema.py:397 Cassandra is primed.