leaking {ssl_closed, _} messages from httpoison/hackney

[tsloughter]

HTTP requests through ex_twilio can result in unmatched {ssl_closed, _} messages leaking. I found this was crashing gen_servers that were making requests with ex_twilio. While the issue may ultimately be in hackney, benoitc/hackney#464, I believe a fix in ex_twilio can be based on this patch to another project that experienced the issue, though I guess some difference since ex_twilio is not using hackney directly: opencensus-beam/oc_google_reporter@bb578a5

[danielberkompas]

I would need more specifics on what specific calls through ExTwilio can result in this error to be able to take action on this.