linked_list_allocator 0.5.0 breaks backward compatibility

[niklasad1]

This PR specifies the version of linked-list-allocator instead of using URL

The following error is generated with version 0.5.0

error[E0599]: no method named `write` found for type `*mut hole::Hole` in the current scope
  --> /home/niklasad1/.cargo/registry/src/github.com-1ecc6299db9ec823/linked_list_allocator-0.5.0/src/hole.rs:29:13
   |
29 |         ptr.write(Hole { size: hole_size, next: None, });
   |             ^^^^^

error[E0599]: no method named `write` found for type `*mut hole::Hole` in the current scope
   --> /home/niklasad1/.cargo/registry/src/github.com-1ecc6299db9ec823/linked_list_allocator-0.5.0/src/hole.rs:286:30
    |
286 |                 unsafe { ptr.write(new_hole) };
    |                              ^^^^^

error: aborting due to 2 previous errors

error: Could not compile `linked_list_allocator`.