Upgrade avro_turf to 1.8.0 to enable schema registry API credentials

[jg197]

Summary
Currently, avro_turf version used does not support the use of schema registry API credentials, made available in dasch/avro_turf@0ed4e6f

What is expected to happen
Bump avro_turf to 1.8.0 and alllow Rimless.avro to be initialised with user, password

Files:
Suggested changes: https://github.com/hausgold/rimless/pull/25/files