Travis-ci is at most on Ubuntu 16.04, that doesn't have openssl >= 1.1,
so disable openssl there.
Semaphore 2.0 was also missing a call to update the package database.
Add another CI as alternative to travis-ci. Test on gcc 6, 7 and 8 on
Ubuntu 18.04. Not sure if this is the best way to define the yml file,
but it works.
The old badge doesn't work. It will be added back later.