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.

META.info

@@ -1,4 +1,5 @@
 {
+  "perl"        : "6.*",
   "name"        : "Pluggable",
   "version"     : "0.1.0",
   "description" : "Helper role to find plugins for your module",