Update watch file format version to 4.

Changes-By: lintian-brush
Fixes: lintian: older-debian-watch-file-standard
See-also: https://lintian.debian.org/tags/older-debian-watch-file-standard.html

debian/changelog

+ruby-bert (1.1.6-3) UNRELEASED; urgency=medium
+
+  * Update watch file format version to 4.
+
+ -- Debian Janitor <janitor@jelmer.uk>  Wed, 16 Jun 2021 05:22:29 -0000
+
 ruby-bert (1.1.6-2) unstable; urgency=medium
 
   * Team upload.

debian/watch

-version=3
+version=4
 https://gemwatch.debian.net/bert .*/bert-(.*).tar.gz