Use extras when running the test server

Make sure that `travis_moto_server.sh` script
actually installs `all` and `server` extras.

travis_moto_server.sh

@@ -1,9 +1,8 @@
 #!/usr/bin/env bash
 set -e
-pip install flask
 # TravisCI on bionic dist uses old version of Docker Engine
 # which is incompatibile with newer docker-py
 # See https://github.com/docker/docker-py/issues/2639
 pip install "docker>=2.5.1,<=4.2.2"
-pip install /moto/dist/moto*.gz
+pip install $(ls /moto/dist/moto*.gz)[server,all]
 moto_server -H 0.0.0.0 -p 5000