Fix the travis build

[talavis]

Describe the pull request:

• [X] Bug fix
• [ ] Functional change
• [ ] New feature
• [ ] Code cleanup
• [X] Build system change
• [ ] Documentation change
• [ ] Language translation

Pull request long description:

Changes in travis made postgres not start on the expected port.

Postgres 10 is now started on the default port (5432) without using docker. The travis script is updated to use the default port.

The build is now tested on Python 3.6, 3.7, and 3.8. The config is done using the standard way in travis instead of defining image and version in a matrix.

Move the installation of coveralls and pylint to test/requirements.txt.