London Mayor Boris Johnson Speaks About U.K. Membership Of European Union [Getty Images]

Boris Johnson, mayor of London, pauses during a Bloomberg Television interview in London, U.K., on Wednesday, Aug. 6, 2014. Johnson said the U.K. could thrive outside the European Union and should be prepared to leave the bloc if Prime Minister David Cameron fails to win sufficient changes to the way it functions. Photographer: Jason Alden/Bloomberg via Getty Images

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