back off Modern::Perl requirement from 2018 to 2015 to fix 5.20 tests

src/perl/bin/lon-tz.pl

@@ -11,7 +11,7 @@
 # pragmas to silence some warnings from Perl::Critic
 ## no critic (Modules::RequireExplicitPackage)
 # This solves a catch-22 where parts of Perl::Critic want both package and use-strict to be first
-use Modern::Perl qw(2018);
+use Modern::Perl qw(2015);
 ## use critic (Modules::RequireExplicitPackage)
 
 use strict;