Update PAT2CAT_cfg.py

This configuration is running in CMSSW_7_1_X.
And if we are using dataset for CMSSW_7_0_X, we can see the message.
Because, the dataset for CMSSW_7_0_X don't has "ak4GenJets".
++++++++++
----- Begin Fatal Exception 21-Jul-2014 14:27:13 CEST-----------------------
An exception of category 'ProductNotFound' occurred while
   [0] Processing run: 1 lumi: 27 event: 7801
   [1] Running path 'outpath'
   [2] Calling event method for module PoolOutputModule/'out'
   [3] Calling produce method for unscheduled module CATJetProducer/'catJets'
   [4] Calling produce method for unscheduled module PATJetProducer/'patJets'
   [5] Calling produce method for unscheduled module GenJetMatcher/'patJetGenJetMatch'
Exception Message:
Principal::getByToken: Found zero products matching all criteria
Looking for type: std::vector<reco::GenJet>
Looking for module label: ak4GenJets
Looking for productInstanceName:

   Additional Info:
      [a] If you wish to continue processing events after a ProductNotFound exception,
add "SkipEvent = cms.untracked.vstring('ProductNotFound')" to the "options" PSet in the configuration.

----- End Fatal Exception -------------------------------------------------
++++++++++