Test thread safety of Avro processor as well

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
Avro: fix problems with NoSuchMethodError

The problem came from the fact that jsonschema2pojo _also_ uses a 1.x version of
Jackson, but a more recent one.

Exclude Avro's version, and it works fine.

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>