Incorrect use of deprecated attribute

[estebank]

rust-lang/rust#56896 is making the correct use of deprecated mandatory in rustc. We can't apply this as a hard error due to the following use in arc-swap:

[INFO] [stderr] error: expected meta item sequence
[INFO] [stderr]    --> /opt/crater/cargo-home/registry/src/github.com-1ecc6299db9ec823/arc-swap-0.3.6/src/lib.rs:500:5
[INFO] [stderr]     |
[INFO] [stderr] 500 |     #[deprecated = "Use upgrade instead"]
[INFO] [stderr]     |     ^^----------------------------------^
[INFO] [stderr]     |       |
[INFO] [stderr]     |       help: use the `note` key: `deprecated(note = "Use upgrade instead")`

[vorner]

Give me an hour or so, I'll release a fix.

[vorner]

A fix should be released in 0.3.7.