AVRO-4271: [ruby] Use json instead of multi_json

[ericproulx]

What is the purpose of the change

This resolves AVRO-4271 by switching the Ruby runtime dependency from
multi_json to the json gem and updating Ruby JSON parsing/generation calls
to use JSON directly. It also removes the stale Yajl.dump call in protocol
message serialization.

Verifying this change

This change is already covered by existing tests. The following targeted tests
were run locally:

• ruby -Ilib:test test/test_schema.rb
• ruby -Ilib:test test/test_schema_normalization.rb
• ruby -Ilib:test test/test_protocol.rb
• ruby -Ilib:test test/test_datafile.rb --name test_differing_schemas_with_complex_objects
• ruby -Ilib:test test/test_io.rb --name '/test_(null|boolean|string|bytes|int|long|float|double)/'

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable