RethinkDB addon does not start

I am building my open source project on Xenial, using the RethinkDB addon. Here’s the relevant travis.yml part:

dist: xenial
language: python
python:
  - 2.7
  - 3.4
  - 3.5
  - 3.6
  - 3.7
addons:
  rethinkdb: 2.3

I can see this in the build logs:

Installing RethinkDB version 2.3
Failed to stop rethinkdb.service: Unit rethinkdb.service not loaded.
$ wget -qO- https://download.rethinkdb.com/apt/pubkey.gpg | sudo apt-key add -v -''
gpg: pub  4096R/5C8DA04A 2017-07-11  RethinkDB Packaging <packaging@rethinkdb.com>
OK
8.80s$ sudo apt-get install -y -o Dpkg::Options::='--force-confnew' rethinkdb=$package_version
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following additional packages will be installed:
  libprotobuf9v5
The following NEW packages will be installed:
  libprotobuf9v5 rethinkdb
0 upgraded, 2 newly installed, 0 to remove and 120 not upgraded.
Need to get 11.4 MB of archives.
After this operation, 40.7 MB of additional disk space will be used.
Get:1 http://download.rethinkdb.com/apt xenial/main amd64 rethinkdb amd64 2.3.6~0xenial [11.0 MB]
Get:2 http://us-east-1.ec2.archive.ubuntu.com/ubuntu xenial/main amd64 libprotobuf9v5 amd64 2.6.1-1.3 [326 kB]
Fetched 11.4 MB in 0s (33.6 MB/s)
Selecting previously unselected package libprotobuf9v5:amd64.
(Reading database ... 124189 files and directories currently installed.)
Preparing to unpack .../libprotobuf9v5_2.6.1-1.3_amd64.deb ...
Unpacking libprotobuf9v5:amd64 (2.6.1-1.3) ...
Selecting previously unselected package rethinkdb.
Preparing to unpack .../rethinkdb_2.3.6~0xenial_amd64.deb ...
Unpacking rethinkdb (2.3.6~0xenial) ...
Processing triggers for libc-bin (2.23-0ubuntu11) ...
Processing triggers for man-db (2.7.5-1) ...
Processing triggers for systemd (229-4ubuntu21.4) ...
Processing triggers for ureadahead (0.100.0-19) ...
Setting up libprotobuf9v5:amd64 (2.6.1-1.3) ...
Setting up rethinkdb (2.3.6~0xenial) ...
The RethinkDB startup service is installed but disabled. To enable it,
follow the instructions in the guide located at http://www.rethinkdb.com/docs/guides/startup/
Processing triggers for libc-bin (2.23-0ubuntu11) ...
Processing triggers for systemd (229-4ubuntu21.4) ...
Processing triggers for ureadahead (0.100.0-19) ...
Installing RethinkDB default instance configuration
Starting RethinkDB v2.3
0.17s$ sudo service rethinkdb start
$ rethinkdb --version
rethinkdb 2.3.6~0xenial (GCC 5.3.1)

However, RethinkDB is not started. I tried debugging the build and I saw the following:

$ sudo service rethinkdb status
# ...status: active (exited)...

In debug mode, I did a manual:

$ sudo service rethinkdb stop
$ sudo service rethinkdb start

and then it finally started, with the status: active (running) status.

What could be the problem and how can this be fixed?