fix ioSerializations usage in scalding 0.9
Merge pull request #852 from epishkin/patch-2

fix ioSerializations usage in scalding-avro/README.md
enable cascading 3.0 traceplans during tests via SCALDING_CASCADING3_DEBUG env variable
Merge pull request #851 from fs111/develop

enable cascading 3.0 traceplans during tests