fastavro is no longer used by default

Description
===========

There seems to have been a bug introduced in #470 that meant that, unless reader schemas are manually specified in the `AvroConsumer` constructor, attempts to use `fastavro` will always fail. (That is, the `reader_schema_obj` variable will be `None`, the attempt to call `to_json()` on it will throw, and the catch-all error handling will mean that it silently falls back to plain Python avro.)

As well as being rather a lot slower, this alters deserialisation behaviour in some cases.

This seems to have first been released in 1.0.0.

Checklist
=========
Please provide the following information:

- [x] confluent-kafka-python and librdkafka version (`confluent_kafka.version()` and `confluent_kafka.libversion()`):
- [ ] Apache Kafka broker version:
- [ ] Client configuration: `{...}`
- [ ] Operating system:
- [ ] Provide client logs (with `'debug': '..'` as necessary)
- [ ] Provide broker log excerpts
- [ ] Critical issue