Elasticsearch.API.AWS doesn't work on OTP 24

[mbklein]

It's an upstream dependency problem, but Sigaws appears to have been unmaintained for some time now, and until this PR gets merged (or Sigaws is forked or reimplmented by someone else), request signing won't work on OTP 24.

[camilohollanda]

@mbklein it looks like someone already forked sigaws with the fix:

https://hex.pm/packages/sigaws_otp_24