Describe the bug

PlatformIO builds started failing with the ArduinoJson package in #74 .

It appears ArduinoJson v.6 is now the considered the latest stable release.
Cpp-Crypto uses ArduinoJson v.5 semantics.

To Reproduce
Steps to reproduce the behavior:

1. attempt to build Cpp-Crypto using PlatformIO

Expected behavior
Cpp-Crypto builds correctly in PlatformIO

Server (please complete the following information):

• OS: macOS
• OS: Linux (CircleCI)

Additional context

This should be resolved by updating Cpp-Crypto to use ArduinoJson v.6 semantics, as well as by updating the repo's corresponding submodule for OS Builds.

Up for discussion is whether we should also force the newest version explicitly in PlatformIO.
e.g. lib_deps = ArduinoJson@6.10.0

ArduinoJson v.5->6 migration guide: https://arduinojson.org/v6/doc/upgrade/