update README.md to add bundle exec when installing spree_i18n
update README.md to add bundle exec when installing spree_i18n

zubietaroberto/spree_i18n,zubietaroberto/spree_i18n,welaika/spree_i18n,priyank-gupta/spree_i18n,spree-contrib/spree_i18n,priyank-gupta/spree_i18n,nietu/spree_i18n,welaika/spree_i18n,milep/spree_i18n,milep/spree_i18n,priyank-gupta/spree_i18n,milep/spree_i18n,nietu/spree_i18n,welaika/spree_i18n,nietu/spree_i18n,zubietaroberto/spree_i18n