Add missing require

[y-yagi]

This gem uses json, but does not require it now. This PR adds it.

In the test, simplecov loads json. So it was passed. If we comment out simplecov, the test would fail like the following.

  1) Error:
JsonRpcHandler::#handle#test_0016_returns an error with the code set to -32700 there is a JSON parse error:
NameError: uninitialized constant JsonRpcHandler::JSON
    lib/json_rpc_handler.rb:54:in 'JsonRpcHandler.handle_json'
    test/json_rpc_handler_test.rb:472:in 'handle_json'
    test/json_rpc_handler_test.rb:284:in 'block (3 levels) in <top (required)>'

  2) Error:
JsonRpcHandler::#handle_json#test_0002_returns nil for notifications:
NoMethodError: undefined method 'to_json' for an instance of Hash
    test/json_rpc_handler_test.rb:449:in 'block (3 levels) in <top (required)>'

  3) Error:
JsonRpcHandler::#handle_json#test_0001_returns a Response object when the request is valid and not a notification:
NoMethodError: undefined method 'to_json' for an instance of Hash
    test/json_rpc_handler_test.rb:441:in 'block (3 levels) in <top (required)>'

  4) Error:
JsonRpcHandler::#handle_json#test_0003_returns an error with the id set to nil when the request is invalid:
NoMethodError: undefined method 'to_json' for an instance of Hash
    test/json_rpc_handler_test.rb:455:in 'block (3 levels) in <top (required)>'