Support later OTP versions

[J0]

Task:

• Add later OTP version to CI
• Fix corresponding dialyzer errors

[kelcecil]

Adding to this... websocket_client is broken on OTP 25 after http_uri:parse/1 was removed in favor of uri_string:parse/1 (included since OTP 21). I've submitted a PR (sanmiguel/websocket_client#73) against websocket_client to make the switch.

[kelcecil]

My PR for websocket_client was merged this morning, so I'll submit a PR in the next day or so to update that dependency here once it's available in hex.