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"        : "Benchmark::Plot",
     "version"     : "*",
     "description" : "Simple benchmark graphing script",