update popit-api to version that strips // in image proxy urls
update popit-api to version that strips // in image proxy urls

For #707

openstate/popit,Sinar/popit,openstate/popit,Sinar/popit,openstate/popit,mysociety/popit,Sinar/popit,openstate/popit,mysociety/popit,mysociety/popit,Sinar/popit,mysociety/popit,mysociety/popit