remove C++11 keywords from 53x

[bendavid]

Fixes issue with gcc462 builds reported in
#5600

[cmsbuild]

A new Pull Request was created by @bendavid (Josh Bendavid) for CMSSW_5_3_X.

remove C++11 keywords from 53x

It involves the following packages:

GeneratorInterface/Pythia8Interface

@vciulli, @cmsbuild, @nclopezo, @bendavid, @thuer can you please review it and eventually sign? Thanks.
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@smuzaffar you are the release manager for this.
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[bendavid]

+1

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_5_3_X IBs unless changes or unless it breaks tests. @smuzaffar can you please take care of it?