Disable s390x builds on Travis-CI (again).
Disable s390x builds on Travis-CI (again).

s390x currently is too unreliable to be part of required CI builds. They can sometimes take hours or days to get queued which makes getting commits through our workflow very time-consuming.

pgbackrest/pgbackrest,pgbackrest/pgbackrest,pgbackrest/pgbackrest,pgmasters/backrest,pgmasters/backrest