update popit-api module to version that has image upload
update popit-api module to version that has image upload

Update the API to be able to accept image uploads and also to update
image properties

Fixes #214

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