Updated to newer Rust

Two main changes:

1. Changed all `pub static` in `avutil52/src/lib.rs` to `pub const`. Firstly, some are copies of each other and as such statics won't work; secondly, I think `const` isn't right. I'm not 100% sure, especially about [`avutil52/src/lib.rs`, line 1142](https://github.com/wackywendell/rust-ffmpeg/blob/0be939e70612819b1d763768e7fe89035beb74d5/avutil52/src/lib.rs#L1142) where its a struct. but it works.
2. I changed `std::str::from_c_str(LIBRARY_license())` to `std::string::from_buf(LIBRARY_license() as *const u8)`. That takes care of the deprecated warning, and when you run `main()` it prints out just fine.