travis: disable OSX for now

travis: disable OSX for now

For weeks we have a problem with timeout on OSX on travis, not sure
how to fix it ... volunteer wanted :-)

 ==> ./bootstrap --prefix=/usr/local/Cellar/cmake/3.19.1 --no-system-libs --paral
 ==> make

 No output has been received in the last 10m0s, this potentially indicates a stalled build or something wrong with the build itself.
 Check the details on how to adjust your build configuration on: https://docs.travis-ci.com/user/common-build-problems/#Build-times-out-because-no-output-was-received

 The build has been terminated

Signed-off-by: Karel Zak <kzak@redhat.com>