auto-update: Fix build failure with GHC 7.4.2

The auto-update 0.1.1.0 failed to install with GHC 7.4.2. A build log is as below:

~~~~
Building auto-update-0.1.1.0...
Preprocessing library auto-update-0.1.1.0...
[1 of 3] Compiling Control.AutoUpdate.Util ( Control/AutoUpdate/Util.hs, dist/build/Control/AutoUpdate/Util.o )

Control/AutoUpdate/Util.hs:16:23:
    Not in scope: type constructor or class `IORef'
~~~~