Loosen Restriction on confluent-kafka to support newer version

[nwmiller]

The latest version of confluent-kafka is now 1.8.2: https://github.com/confluentinc/confluent-kafka-python/releases/tag/v1.8.2.

confluent_kafka_helpers/setup.py

Line 15 in 7c8744f

'confluent-kafka>=1.0.0,<=1.7.0',

If possible, allowing installs of newer versions of the package would be ideal. Perhaps compatible releases up until the next major release 2.0.0 could be allowed.