Use new allocator API

[Woyten]

libtock-rs cannot be built with the newest nightly due to rust-lang/rust#49669.

There is a PR which will update the linked_list_allocator: rust-osdev/linked-list-allocator#11

TODOs

• Update linked_list_allocator version in Cargo.toml and make the build great again.

[Woyten]

I made it working on https://github.com/torfmaster/libtock-rs/tree/nightly-rust. Travis build: https://travis-ci.org/torfmaster/libtock-rs/builds/368787109

The changes aren't critical, so I suggest to leave the issue open until we have a good reason to update the Rust toolchain.