Update Travis to use latest Firefox dev edition in testing
Update Travis to use latest Firefox dev edition in testing

mozilla-services/pageshot,mozilla-services/pageshot,mozilla-services/pageshot,mozilla-services/screenshots,mozilla-services/screenshots,mozilla-services/screenshots,fzzzy/pageshot,fzzzy/pageshot,mozilla-services/pageshot,fzzzy/pageshot,fzzzy/pageshot,mozilla-services/screenshots