Remove coveralls test dependency

[aaronrenner]

I tried running mix test on elixir 1.15.6, Erlang 26.1.1, Ubuntu jammy and received the following errors when compiling dependencies.

===> Analyzing applications...
===> Compiling mimerl
==> ssl_verify_fun
Compiling 7 files (.erl)
src/ssl_verify_fingerprint.erl:15:14: can't find include lib "public_key/include/public_key.hrl"
%   15| -include_lib("public_key/include/public_key.hrl").
%     |              ^

src/ssl_verify_fun_cert_helpers.erl:13:14: can't find include lib "public_key/include/public_key.hrl"
%   13| -include_lib("public_key/include/public_key.hrl").

It turns out this can be fixed by upgrading ssl_verify_fun, but I'm not really sure that coveralls is even needed anymore since mix test.coverage was introduced in Elixir 1.11.

What do you think about removing coveralls as a dependency so this project is easier to run in newer versions of Elixir?