Update Coveralls image in README.
Update Coveralls image in README.

jzohrab/supermarket,rafaelmagu/supermarket,jzohrab/supermarket,robbkidd/supermarket,rafaelmagu/supermarket,juliandunn/supermarket,juliandunn/supermarket,nellshamrell/supermarket,robbkidd/supermarket,robbkidd/supermarket,dpnl87/supermarket,chef/supermarket,juliandunn/supermarket,juliandunn/supermarket,leftathome/supermarket,rafaelmagu/supermarket,chef/supermarket,tas50/supermarket,rafaelmagu/supermarket,jzohrab/supermarket,nellshamrell/supermarket,chef/supermarket,dpnl87/supermarket,leftathome/supermarket,tas50/supermarket,jzohrab/supermarket,dpnl87/supermarket,leftathome/supermarket,dpnl87/supermarket,chef/supermarket,nellshamrell/supermarket,chef/supermarket,leftathome/supermarket,tas50/supermarket,nellshamrell/supermarket,robbkidd/supermarket,tas50/supermarket