Add tests for Postgresql hooks

[cnouguier]

To enhance the global test coverage, it is needed to add some test for the Postgresql hooks. It is also needed to update the Travis configuration in order to instanciante a Postgresql database. See the following links:

• http://mattmakesmaps.com/blog/2014/01/11/configure-postgis-with-travis-ci/
• https://docs.travis-ci.com/user/database-setup/#PostgreSQL