Add license field to META6.json

also write a test that META6.json is a valid JSON file.
Hey, we have a JSON parser right there, why not use it?

Closes #29.

Author: moritz

META6.json

@@ -4,6 +4,7 @@
     "version"     : "*",
     "description" : "A minimal JSON (de)serializer",
     "depends"     : [ ],
+    "license":    "Artistic-2.0",
     "provides"    : {
         "JSON::Tiny"            : "lib/JSON/Tiny.pm",
         "JSON::Tiny::Actions"   : "lib/JSON/Tiny/Actions.pm",

t/06-meta-valid.t

@@ -0,0 +1,18 @@
+use v6;
+use Test;
+
+use JSON::Tiny;
+
+my $contents = slurp 'META6.json';
+ok $contents, 'can read META6.json';
+my %decoded;
+lives-ok {
+    %decoded = from-json($contents);
+}, 'can decode META6.json';
+
+ok %decoded, 'META6.json is not empty';
+for <perl name description license provides source-url> -> $key {
+    ok %decoded{$key}.defined, "key $key present";
+}
+
+done-testing;