Bump lower bound for base

We only test with GHC 8.4+ so it's unlikely this builds with older base
(Semigroup stuff …).

xmonad-contrib.cabal

@@ -49,7 +49,7 @@ flag pedantic
   manual:      True
 
 library
-    build-depends: base >= 4.9 && < 5,
+    build-depends: base >= 4.11 && < 5,
                    bytestring >= 0.10 && < 0.12,
                    containers >= 0.5 && < 0.7,
                    directory,