Update to `brotli2`.

[Protryon]

The usage of the brotli crate creates an ABI incompatibility with brotli2, which in turn creates ABI incompatibility with actix-web unless compression is disabled. Alternatively, one of the two projects could mangle C names of statically linked library.

[Nemo157]

Moving to brotli from brotli2 was done to make building the crate easier by avoiding unnecessary C dependencies (see #39).

Really, the issue here is that brotli doesn't distinguish between being used as a Rust dependency and a static library, exporting #[no_mangle] functions in both cases. Opened dropbox/rust-brotli#46 to see if this can be fixed there.

[Darkspirit]

Also, actix/actix-web@1d12ba9 should be reverted and fixed.