Does not compile on `*-linux-android` triple

[ScottBot10]

The get_minecraft_dir function doesn't have any return value if the OS is android.

https://github.com/gorilla-devs/libium/blob/01d3aebe6c21e93376143b49d4bbd3e0325cb411/src/lib.rs#L42-L58

[theRookieCoder]

Fixed by #474