Improve logic in PostgresVersion.pm

Handle the situation where perl swaps the order of operands of
the comparison operator. See `perldoc overload` for details:

The third argument is set to TRUE if (and only if) the two
operands have been swapped. Perl may do this to ensure that the
first argument ($self) is an object implementing the overloaded
operation, in line with general object calling conventions.

src/test/perl/PostgresVersion.pm

@@ -110,10 +110,12 @@ sub new
 #
 sub _version_cmp
 {
-	my ($a, $b) = @_;
+	my ($a, $b, $swapped) = @_;
 
 	$b = __PACKAGE__->new($b) unless blessed($b);
 
+	($a, $b) = ($b, $a) if $swapped;
+
 	my ($an, $bn) = ($a->{num}, $b->{num});
 
 	for (my $idx = 0;; $idx++)