specify a osx 10.8.5 specific functional version of therubyracer...

This is a version of therubyracer that works on osx 10.8.5... ruby 1.9.3p392 (2013-02-22 revision 39386) [x86_64-darwin12.3.0]
per: http://stackoverflow.com/questions/10905820/gem-install-therubyracer-fails-on-mac-os-x-lion

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