Use SPDX identifier in license field of META.info

Use the standardized identifier for the license field.
For more details see https://design.perl6.org/S22.html#license

Author: samcv

META.info

@@ -1,9 +1,13 @@
 {
-	"name" : "Imlib2",
-	"version" : "0.0.1",
-	"description" : "Perl 6 interface to the Imlib2 image library.",
-	"author" : "Henrique Dias",
-	"depends" : [ "NativeCall", "LibraryMake" ],
-	"source-type" : "git",
-	"source-url" : "git://github.com/mrhdias/perl6-Imlib2.git"
-}
+  "name": "Imlib2",
+  "source-url": "git://github.com/mrhdias/perl6-Imlib2.git",
+  "source-type": "git",
+  "author": "Henrique Dias",
+  "depends": [
+    "NativeCall",
+    "LibraryMake"
+  ],
+  "license": "Artistic-2.0",
+  "version": "0.0.1",
+  "description": "Perl 6 interface to the Imlib2 image library."
+}