Update poison

[MasashiYokota]

Thank you for this awesome project. As I found that the poison version was still v3.1.0, I've updated poison v4.0.1 which is the latest version.

I would appreciate it very much if you could point out anything to me.

[MasashiYokota]

@stavro I am afraid that you are having a hectic week, but I found that my PR's CI failed caused by the CI's poison version mismatch. (I've checked that mix compile and mix test passed in my environment.)

I'd like to resolve this issue, but I couldn't find any semaphoreci's configuration fiies (eg. .semaphore/semaphore.yml etc.) Do you have any ideas to resolve it? I look forward to hearing from you. Thank you.

[MasashiYokota]

@stavro I'm terribly sorry to disturb you again, but I'd like you to confirm my PR.

[williamweckl]

Any news on this? I think elixir version needs to be updated at CI.