By Joice Alves LONDON (Reuters) -Sterling slumped against to a two-year low against the dollar on Tuesday as a crisis in British Prime Minister Boris Johnson’s government compounded pressure on a currency already reeling from recession fears and a resurgent greenback. Finance minister Rishi Sunak’s resigned minutes after the health minister stepped down saying he had lost confidence in Johnson’s ability to govern in the national interest. The resignations could be the final blow for Johnson’s premiership after he had tried to apologise for the latest scandal to dog his almost three years in of…