Add ppc64le to Travis-CI build matrix.
Add ppc64le to Travis-CI build matrix.

apt.postgresql.org provides packages for ppcle64 so it's important that we support it.

Rearrange jobs a bit based on current runtimes and importance. Also reduce the number of tests run for arm64 since it is slower than other architectures.

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