Fix travis to restart mongodb

Author: davimacedo

.travis.yml

@@ -39,7 +39,7 @@ before_install:
 - npm install -g greenkeeper-lockfile@1
 before_script:
 - echo "replSet = rs0" | sudo tee -a /etc/mongodb.conf
-- sudo service mongodb restart
+- sudo systemctl restart mongod
 - sleep 20
 - mongo --eval 'rs.initiate()'
 - sleep 20