Follow updates about pgbench of PostgreSQL.
Follow updates about pgbench of PostgreSQL.

The commit [81134af3ec09d67043833f8d614fd688f17cb213] moves
pgbench from contrib/ to src/bin.Thus, we can remove code to
build contrib/pgbench from .travis.yml.

bwtakacy/prev_pg_rman_repo,bwtakacy/prev_pg_rman_repo