Bump version (and xmonad dependency) to 0.12

This makes a breakage due to missing patches in core a bit more obvious.
Previously you would have a build failure regarding some missing identifiers
(def re-exported by XMonad from Data.Default), while after applying this patch
it will be clear that xmonad-core needs to be updated.

xmonad-contrib.cabal

@@ -1,5 +1,5 @@
 name:               xmonad-contrib
-version:            0.11
+version:            0.12
 homepage:           http://xmonad.org/
 synopsis:           Third party extensions for xmonad
 description:
@@ -73,7 +73,7 @@ library
         extensions: ForeignFunctionInterface
         cpp-options: -DXFT
 
-    build-depends:      mtl >= 1 && < 3, unix, X11>=1.6.1 && < 1.7, xmonad>=0.11   && < 0.12, utf8-string
+    build-depends:      mtl >= 1 && < 3, unix, X11>=1.6.1 && < 1.7, xmonad>=0.12   && < 0.13, utf8-string
 
     if true
         ghc-options:    -fwarn-tabs -Wall