Declare dependancy on eunit in mix file for elixir 1.15 compatibility

[howleysv]

Without this config, compiling on elixir 1.15 errors with

src/bucs.erl:3:14: can't find include lib "eunit/include/eunit.hrl"
%    3| -include_lib("eunit/include/eunit.hrl").
%     |              ^