Linked-list-allocator v0.6.6 causes Travis-CI to fail

[gendx]

The release of version v0.6.6 earlier today (rust-osdev/linked-list-allocator#20) causes Travis-CI build to fail: https://travis-ci.org/google/OpenSK/builds/645900468

    Checking linked_list_allocator v0.6.6
error[E0432]: unresolved import `alloc::alloc::AllocRef`
  --> /home/travis/.cargo/registry/src/mygithub.libinneed.workers.dev-1ecc6299db9ec823/linked_list_allocator-0.6.6/src/lib.rs:14:20
   |
14 | use alloc::alloc::{AllocRef, AllocErr, Layout};
   |                    ^^^^^^^^ no `AllocRef` in `alloc`
error: aborting due to previous error

See also rust-osdev/linked-list-allocator#21.

This will likely be fixed by increasing the rust-toolchain version.