Add gitter to Travis CI config. Remove slack.
Add gitter to Travis CI config. Remove slack.

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