README: adding —pre to pip since package is still beta

Since the package is still marked as beta, pip complains when running `pip install cassandra-driver`:

```shell
Downloading/unpacking cassandra-driver
  Could not find a version that satisfies the requirement cassandra-driver (from versions: 1.0.0b5, 1.0.0b6, 1.0.0b7)
Cleaning up...
No distributions matching the version for cassandra-driver
Storing complete log in /home/danielesalatti/.pip/pip.log
```

I added `--pre` to force pip to install the package anyway.