Bump up waiting for MongoDB from 15s to 20s

I've noticed that `mongo --eval 'db.version()'` has been failing fairly
regularly in the last few weeks. Hopefully that extra 5s is enough.

Author: wojcikstefan

.travis.yml

@@ -39,7 +39,7 @@ matrix:
 
 before_install:
 - bash .install_mongodb_on_travis.sh
-- sleep 15  # https://docs.travis-ci.com/user/database-setup/#MongoDB-does-not-immediately-accept-connections
+- sleep 20  # https://docs.travis-ci.com/user/database-setup/#mongodb-does-not-immediately-accept-connections
 - mongo --eval 'db.version();'
 
 install: