Remove envy dependency

The envy app doesn't add real value. 
It is only used in a single place and can easily replaced.
I suggest to remove it to simplify the dependencies.

``` erlang
Defaults = envy:get(mini_s3, s3_defaults, list),
```

``` erlang
Defaults = case application:get_env(mini_s3, s3_defaults) of
    Def when is_list(Def) -> Def;
    _ -> []
end,
```
