Fedora 26 is EOL, replace it by Fedora 28 in travis CI
Fedora 26 is EOL, replace it by Fedora 28 in travis CI

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

projectatomic/osbs-client,projectatomic/osbs-client,DBuildService/osbs-client,DBuildService/osbs-client