Bump minimum rustc version to 1.21.0

[messense]

Should fix CI

error: associated constants are experimental (see issue #29646)
--> /home/travis/.cargo/registry/src/github.com-1ecc6299db9ec823/crossbeam-epoch-0.4.0/src/internal.rs:52:5
|
52 | const COLLECT_STEPS: usize = 8;
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[messense]

error: use of unstable library feature 'ord_max_min' (see issue #25663)
--> /home/travis/.cargo/registry/src/github.com-1ecc6299db9ec823/crossbeam-deque-0.3.0/src/lib.rs:667:27
|
667 | b.wrapping_sub(t).max(0) as usize
| ^^^
error: aborting due to previous error

rust-lang/rust#44593

Guess it needs 1.22 then.

https://github.com/rust-lang/rust/blob/c90f68224b069f5bb2a80e30e2737e4bb17c1466/src/libcore/cmp.rs#L460

1.21.0 actually.

[seanmonstar]

Unfortunate, but thanks!