Update Travis config to run on Fedora 28
Master branch was updated to run the latest release of Fedora (30).
This makes Fedora 28 to be used to test ipa-4-7 because it has
Python 2 included.
Team discussed that we should go for Fedora 28 because it ships a
similar Python version than RHEL 8's platform-python.
Signed-off-by: Armando Neto <abiagion@redhat.com>
Reviewed-By: Christian Heimes <cheimes@redhat.com>