removes reference in imap iterator_traits
removes reference in imap iterator_traits

jkhoogland/cppitertools,jkhoogland/cppitertools,tempbottle/cppitertools,tempbottle/cppitertools,ryanhaining/cppitertools,tempbottle/cppitertools,jkhoogland/cppitertools,ryanhaining/cppitertools,ryanhaining/cppitertools