correct membership tense

Handle cases when we have a start and an end date to prevent things like
current memberships using 'will be a member of'

Fixes #663
add alternative image URL to match API schema

The API uses a different URL scheme to identify images so add an
alternative one for the UI so the two can match up.

Also update shrinkwrap to point at the version of the API that uses the
same URLs

For mysociety/popit-api#84