Add mandatory "perl" META field

The `perl` field specifies the minimal perl version for which this distribution can be installed and is a mandatory field. The value of `"6.*"` indicates any version suffices.

It is recommended to use [Test::META](https://modules.perl6.org/repo/Test::META) module as an author test, to catch any issues with the META file.

META6.json

@@ -1,4 +1,5 @@
 {
+    "perl" : "6.*",
     "name" : "X11::Xlib::Raw",
     "version" : "*",
     "description" : "Xlib binding using NativeCall",
@@ -8,4 +9,4 @@
     },
     "repo-type" : "git",
     "source-url" : "git://github.com/slobo/Perl6-X11-Xlib-Raw.git"
-}
+}