Fedora 26 is EOL, add Fedora 28 to travis CI
Fedora 26 is EOL, add Fedora 28 to travis CI

Signed-off-by: Clement Verna <42821abe07dd2b40ac4015e34f880ddb708f32d6@tutanota.com>

fr34k8/atomic-reactor,projectatomic/atomic-reactor,fr34k8/atomic-reactor,projectatomic/atomic-reactor,DBuildService/atomic-reactor,DBuildService/atomic-reactor