Add unit declarator to module declarations

As of Rakudo 2015.05, the `unit` declarator is required before using
`module`, `class` or `grammar` declarations (unless it uses a block).  Code
still using the old blockless semicolon form will throw a warning. This
commit stops the warning from appearing in the new Rakudo.

lib/Netstring.pm6

@@ -1,6 +1,6 @@
 use v6;
 
-module Netstring;
+unit module Netstring;
 
 proto to-netstring ($) is export {*}
 proto to-netstring-buf ($) is export {*}