Set Travis CI docker geoserver instance for integration tests

[eblondel]

All docker images tested work, but no sample data configured. To investigate if there is a way to configure basic sample data in Geoserver, otherwise all tests will need to be revised.

Known docker geoserver images

https://github.com/oscarfonts/docker-geoserver - question sent to owner (oscarfonts/docker-geoserver#6)
https://github.com/kartoza/docker-geoserver
https://github.com/thinkWhere/GeoServer-Docker
https://bitbucket.org/ololoteam/geoserver-docker