KNOX-2830 - Disabled Travis CI integration

[smolnar82]

What changes were proposed in this pull request?

Renamed .travis.yml to .travis.yml.disabled so that Travis CI won't trigger Knox builds in the future. I did not remove this file to preserve historical changes:

$ git log --oneline --follow .travis.yml.disabled 
a904c048d (HEAD -> KNOX-2830, origin/KNOX-2830) KNOX-2830 - Disabled Travis CI integration
28159cfb2 Displaying disk usage on Travis CI nodes
b83e3315e KNOX-2659 - Travis build failure: '/usr/bin/mvn': Operation not permitted. (#491)
8c135d425 KNOX-2639. Build instability because of maven artifact download failure (#475)
875da3232 KNOX-2629 - Fix new shellcheck errors (#465)
237885977 KNOX-2535 - Add a job to build and test on ARM64 Graviton2 node at TravisCI (#402)
4179fee8f KNOX-2115 - Improve .travis.yml (#187)
a1d86ae03 KNOX-1816 - Added shellcheck validation to our build optionally and fixed issues shellcheck already found (#114)
ca1cdd4eb KNOX-1709 - Allow tests to run in parallel
ef7afca67 Only test JDK 8 and 11
5181adb5f KNOX-1458 - Support JDK 9/10/11
03b1354a1 Fix JDK8 docker
3098fadba Disable JDK 9, 10, 11
1526f58b2 Fix travis due to Debian JDK bug 911925
5bf7bc5d0 KNOX-1441 - Enable spotbugs for static build checking

How was this patch tested?

Submitted this change as a GitHub PR and checking if Travis CI is actually disabled (i.e. on the GitHub Actions verification should be executed with this PR).