Disable erroring TravisCI build

[bhess]

Since TravisCI is failing since 2 weeks (#1888) and the ticket there is still under investigation, I'd suggest to disable it temporarily to not longer interrupt the liboqs CI pipeline.

• Does this PR change the input/output behaviour of a cryptographic algorithm (i.e., does it change known answer test values)? (If so, a version bump will be required from x.y.z to x.(y+1).0.)
• Does this PR change the list of algorithms available -- either adding, removing, or renaming? Does this PR otherwise change an API? (If so, PRs in fully supported downstream projects dependent on these, i.e., oqs-provider will also need to be ready for review and merge by the time this is merged.)

[baentsch]

Absolutely agree -- big surprise as this is actually implementing part of the proposal in #1745 (comment) :). The completion on this line item would be creation of an issue to add Travis back eventually (if someone cares about those platforms).