Fix jason, ex_aws deps conflict

[suranyami]

• Updated jason version to 1.4.
• ex_aws 2.4 now includes qldb.session so don't need to use the git dependency.
• Fixed @type and @spec definitions so they use more recent map syntax, optional() and required() key specs.
• Updated README to show ex_aws_qldb instead of ex_aws_lambda.

[suranyami]

Hey @georgelima do you need someone to help maintain this? I'm happy to help: bring things up to date at least. I've got a current need to use QLDB, so it's a good time for me to get involved… enlightened self-interest and all that.

Alternatively, I can keep working on my fork, but it seems a shame.