Warn on invalid certainty values

Change-Id: Ia6ec333036d25422b39cb72e7911da0c6571b95a
diff --git a/Makefile.PL b/Makefile.PL
index 6032c0d..ff1de01 100644
--- a/Makefile.PL
+++ b/Makefile.PL
@@ -45,6 +45,7 @@
     'Archive::Tar'    => 2.24,
     'Clone'           => 0.45,
     'List::Util'      => 1.45,
+    'Scalar::Util'    => 1.60,
   },
   MIN_PERL_VERSION => '5.016',
   test => {