make dotenv_elixir 0.12.x compatible

- the __FILE__ has been replaced by __MODULE__.file
- the default argument // raises a deprecation warning
- one test was failing dotenv_app_test: "fetching a var" I'm not sure why it was failing, my guess is calling reload! without any params reloads the current env_paths. and depending on the order of the tests, that can be an other file than the default one.

I also add 

teardown do
  System.put_env { "APP_TEST_VAR": nil, "FOO_BAR": nil, "MISSING": nil }
end

As I usually try to leave the env in the same state as it was before the tests.