Fix mongo config

We need to update to the new mongo 3.x api since the travis build is currently broken. See: https://travis-ci.org/mailvelope/keyserver/builds/282576101#L845

.travis.yml

@@ -10,7 +10,7 @@ services:
   - mongodb
 
 before_script:
-  - mongo test_db --eval 'db.addUser("travis", "test");'
+  - mongo test_db --eval 'db.createUser({user:"travis",pwd:"test",roles:["readWrite"]});'
 
 before_deploy:
   - npm run release