Bump version to 2.6.1

UTC by default (#30) is an important enough improvement/fix
that it warrants an immediate patch release.

This should not break any code because RFC 3339 requries
an explicit timezone for non-UTC times, and switching the timezone
should not break any conformant parser.

Author: Techcable

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "slog-json"
-version = "2.6.0"
+version = "2.6.1"
 edition = "2018"
 authors = ["Dawid Ciężarkiewicz <dpc@dpc.pw>"]
 description = "JSON drain for slog-rs"