Cleanup: use subroutine signatures, remove deprecated methods.

The signatures feature has been "experimental" since Perl 5.20 (May 2014), but
expected to stay. After 8 years I'm ready to take the risk :)

Have added Perl v5.28 (June 2018) as the minimum requirement, even though the
current revbank should work with 5.20, to see if this bothers any users. Perl
v5.28 is in Debian "buster", which is now oldstable.

lib/RevBank/Plugin.pm

@@ -1,12 +1,16 @@
 package RevBank::Plugin;
-use strict;
+
+use v5.28;
+use warnings;
+use feature qw(signatures);
+no warnings qw(experimental::signatures);
+
 require RevBank::Global;
 
-sub new {
-    my ($class) = @_;
+sub new($class) {
     return bless { }, $class;
 }
-sub command {
+sub command($self, $cart, $command, @) {
     return RevBank::Global::NEXT();
 }