Cache dependencies downloaded via pip on Travis CI
Follow Travis CI build config validation warnings and infos
Call Discord webhook after Travis CI build
Test on Travis CI with PostgreSQL v11 instead of v9.6
v11 is the default PostgreSQL version that comes with Debian "Buster" 10.
Install only test but not other development dependencies for CI test runs
Avoid redundant installation of unpinned pytest version
Use merged requirements file to obtain test dependencies on Travis CI
Use Ubuntu Bionic (instead of Trusty) on Travis CI to get Pythion 3.7 working
Raise required Python version to 3.7 (from 3.6)
This makes data classes available.
Adjust Travis CI config to explicitly install dependencies before running tests
Adjust Travis CI config to run `pytest` now
Raise required Python version to 3.6 (from 3.5)
Use 'psycopg2' binary installed via pip on Travis CI, too
Raise minimum required Python version from 3.4 to 3.5
Upgrade pytest on Travis CI to get v3.3 rather than v3.0 provided by default
PostgreSQL v9.6 is only available as of Ubuntu Trusty, but not yet on Ubuntu Precise
Upgrade to PostgreSQL v9.6 now that it became available in Debian's `jessie-backports` repository
Update pip itself before installing other packages with it.
Adjusted overlooked Travis CI configuration file to moved files.