Mark rayon as a public dependency

[kornelski]

Tests on the main branch fail. It seems that rayon is intentionally exposed.

image/src/lib.rs

Lines 187 to 188 in e176cd4

	#[cfg(feature = "rayon")]
	pub use crate::buffer_par::*;

[HeroicKatora]

Yep, I believe it's intentional as well. And since it's v1 and optional anyways there's not much wrong doing so.