Fix URL to documentation.

[rbtcollins]

The docs at https://docs.rs/failure/0.1.5/failure/ were linking to the pre-nursery versions of the docs from withoutboats; this is harmful as the recommended use has evolved.

[rbtcollins]

The failure is in master too - error: use of deprecated item 'std::sync::atomic::ATOMIC_USIZE_INIT': the `new` function is now preferred --> src/backtrace/internal.rs:5:38 | 5 | use std::sync::atomic::{AtomicUsize, ATOMIC_USIZE_INIT, Ordering}; | ^^^^^^^^^^^^^^^^^

[palfrey]

This should build ok now #303 is in