Fix CompileError due to default values and multiple clauses

On Elixir 0.14.3, compilation fails with the following message:

    == Compilation error on file lib/chronos.ex ==
    ** (CompileError) lib/chronos.ex:165: def days_ago/2 has default values
       and multiple clauses, define a function head with the defaults
        (elixir) src/elixir_def.erl:336: :elixir_def.store_each/8
        (elixir) src/elixir_def.erl:107: :elixir_def.store_definition/9
        lib/chronos.ex:165: (module)
        (stdlib) erl_eval.erl:657: :erl_eval.do_apply/6
        (elixir) src/elixir.erl:170: :elixir.erl_eval/3

This is due to a change from elixir-lang/elixir#2467