[Security] potential issue with jsonpickle

Hey sefeki,

aws-xray-sdk-python only applies API encode from jsonpickle, see: https://github.com/aws/aws-xray-sdk-python/search?q=jsonpickle, so currently it's safe to use aws-xray-sdk-python. But since this already raised the awareness of security issue, we will be working to replace it with another feasible dependency to serialize data in the near future to mitigate this security warning.