Fix compile error due to nonexisting `atom_to_binary`

Unfortunately compiling dialyxir with elixir-0.14.2 fails as follows:
```shell
$ mix compile
== Compilation error on file lib/dialyzer.plt.ex ==
** (CompileError) lib/dialyzer.plt.ex:77: function atom_to_binary/1 undefined
    (stdlib) lists.erl:1336: :lists.foreach/2
    (stdlib) erl_eval.erl:657: :erl_eval.do_apply/6
    (elixir) src/elixir.erl:163: :elixir.erl_eval/2
    (elixir) src/elixir.erl:156: :elixir.eval_forms/4
    (elixir) src/elixir_lexical.erl:17: :elixir_lexical.run/2
    (elixir) src/elixir.erl:163: :elixir.erl_eval/2
```
This pull request simply replaces `atom_to_binary` with `Atom.to_string` as explained in https://github.com/elixir-lang/elixir/releases.

Just for reference:
- `Atom.to_string` is defined in [this commit](https://github.com/elixir-lang/elixir/commit/deb0191a2144dd890195081110e942437a963341).
- `atom_to_binary` is removed in [this commit](https://github.com/elixir-lang/elixir/commit/feb9cb645ed6f4199979e5fb9ff832b83c701a25).

Thanks!