Revert the dev-version bump.

Stop using --dev-version, in line with cargo-release defaults and
common practice.

Author: sunfishcode

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "io-lifetimes"
-version = "0.6.1-alpha.0"
+version = "0.6.0"
 description = "A low-level I/O ownership and borrowing library"
 authors = ["Dan Gohman <dev@sunfishcode.online>"]
 license = "Apache-2.0 WITH LLVM-exception OR Apache-2.0 OR MIT"