Fix configuring cassette_library_dir

[wojtekmach]

• when user of the app calls cassette_library_dir/2, that function is
  actually being called twice. 1st time because of
  ConfigLoader.load_defaults and 2nd time with the custom dir. Thus,
  calling File.mkdir_p inside cassette_library_dir/2 with a custom
  dir, always created 2 dirs: the default one and the custom one
• it looks like this call isn't needed here, because ExVCR.JSON.save/2
  creates a directory when it doesn't exist
• Use File.mkdir_p! since we don't do explicit error handling

[coveralls]

Coverage decreased (-0.02%) to 93.059% when pulling a990cdc on wojtekmach:fix_cassette_library_dir_config into e8ce74e on parroty:master.

[parroty]

Thanks!. It's not well structured and messed at adding the default setting 😓