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 a990cdce532882ec93a5537fdf53faacdabf5786 on wojtekmach:fix_cassette_library_dir_config into e8ce74edac235532eecce7703d5c746572f18c07 on parroty:master.

[parroty]

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