fix karma.conf.js to run firefox only on travis
fix karma.conf.js to run firefox only on travis

opbeat/opbeat-angular,jahtalab/opbeat-js,opbeat/opbeat-react,opbeat/opbeat-js-core,opbeat/opbeat-angular,opbeat/opbeat-react,opbeat/opbeat-angular,opbeat/opbeat-js-core,jahtalab/opbeat-js,opbeat/opbeat-react,jahtalab/opbeat-js