Update hackney to fix sslv3 reference on OTP 23

[fhunleth]

This fixes the following error when using OTP 23:

 ** (Protocol.UndefinedError) protocol String.Chars not implemented for {:options, {:sslv3, {:versions, [:"tlsv1.2", :
        (elixir 1.10.3) lib/string/chars.ex:3: String.Chars.impl_for!/1
        (elixir 1.10.3) lib/string/chars.ex:22: String.Chars.to_string/1
        lib/excoveralls/poster.ex:58: ExCoveralls.Poster.send_file/2
        lib/excoveralls/poster.ex:13: ExCoveralls.Poster.execute/2
        (mix 1.10.3) lib/mix/tasks/test.ex:484: Mix.Tasks.Test.do_run/3
        (mix 1.10.3) lib/mix/task.ex:330: Mix.Task.run_task/3
        lib/mix/tasks.ex:54: Mix.Tasks.Coveralls.do_run/2
        (mix 1.10.3) lib/mix/task.ex:330: Mix.Task.run_task/3
        (mix 1.10.3) lib/mix/cli.ex:82: Mix.CLI.run_task/2
        (elixir 1.10.3) lib/code.ex:926: Code.require_file/2

The error is due to hackney previously adding sslv3 to the ssl_options
when they aren't specified and OTP 23 removing support for sslv3.
Hackney 1.16.0 fixes this.

[coveralls]

Coverage remained the same at 89.823% when pulling 23b503b on fhunleth:update-hackney into 6332f40 on parroty:master.

[parroty]

Thanks!