Update rust dependency to rust-nightly-bin (avoid compatibility issues)

Hello !

First of all, thanks for your AUR package and for maintaining this script :).

I noticed that the package is not working any more on a sample rust project because of version compatibility issues (option print-file-name). Here is an asciinema of this bug (on a sample project).

https://asciinema.org/a/11306

So I edited PKGBUILD to update community/rust to aur/rust-nightly-bin before installing aur/cargo-nightly-bin. When I tried to build the sample project again with this setup, no more print-file-name issue. Below is the result.

http://asciinema.org/a/11311

Anyway, replacing a community dependency with an AUR one is not a painless decision. Moreover, download time and compilation time for rust-nightly-bin could be a lot longer than installation of community/rust. But i really think a nightly would be better depending on another nightly.

If you think it's not a good idea, maybe leave choice to user is another way to make it. 

Let me know what you think about it. Thanks for reading :).