travis: skip builds on -maint branches

travis: skip builds on -maint branches

For patches pushed upstream we want builds run on master branch, but
don't want them run on the -maint branches, as we are not keeping the
.travis.yml file on stable branches updated wrt latest needs of travis
CI platforms.

We can't just whitelist 'master' though, because that will prevent
developers triggering their own private travis builds. So we just
blacklist *-maint, since developers will typically use named feature
branches for any work.

Reviewed-by: Pavel Hrdina <phrdina@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>