- Updated version of spray-json to depend upon in the core sub-project.
- Bumped the patch version number in the top level build definition.
- Added a convenience method (for building protocols) AvroType.dependentNamedTypes.
- Refactored the implementation of AvroType.dependsOn in terms of the new AvroType.dependentNamedTypes (named types now "dependUpon" themselves, which is a semantic difference).  Updated tests accordingly.