Use /bin/bash for contrib/pg_upgrade tests
Use /bin/bash for contrib/pg_upgrade tests

To support Ubuntu, we saw this error:
```
make -C contrib/pg_upgrade check

/bin/sh test_gpdb.sh -C -s -o ../../gpAux/gpdemo/datadirs/ -b /usr/local/gpdb/bin

test_gpdb.sh: 38: test_gpdb.sh: [[: not found
test_gpdb.sh: 68: test_gpdb.sh: pushd: not found
test_gpdb.sh: 69: test_gpdb.sh: time: not found
```

There is an important difference between /bin/sh and /bin/bash in
Ubuntu, so we force use of bash here. This commit assumes that /bin is
in path, such that /bin/bash is found.

Signed-off-by: Goutam Tadi <6458d0db8cef194a4536e82c45165efb9e4e85c5@pivotal.io>
Signed-off-by: Larry Hamel <c5a53acaa5da6b424e2bd28a0587d04439c72a22@pivotal.io>

edespino/gpdb,yuanzhao/gpdb,edespino/gpdb,lisakowen/gpdb,xinzweb/gpdb,lisakowen/gpdb,Chibin/gpdb,yuanzhao/gpdb,edespino/gpdb,lisakowen/gpdb,adam8157/gpdb,ashwinstar/gpdb,xinzweb/gpdb,jmcatamney/gpdb,yuanzhao/gpdb,xinzweb/gpdb,Chibin/gpdb,xinzweb/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,50wu/gpdb,edespino/gpdb,jmcatamney/gpdb,50wu/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,adam8157/gpdb,50wu/gpdb,yuanzhao/gpdb,ashwinstar/gpdb,Chibin/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,xinzweb/gpdb,jmcatamney/gpdb,lisakowen/gpdb,yuanzhao/gpdb,Chibin/gpdb,greenplum-db/gpdb,Chibin/gpdb,yuanzhao/gpdb,edespino/gpdb,xinzweb/gpdb,ashwinstar/gpdb,Chibin/gpdb,greenplum-db/gpdb,50wu/gpdb,edespino/gpdb,lisakowen/gpdb,lisakowen/gpdb,Chibin/gpdb,adam8157/gpdb,jmcatamney/gpdb,Chibin/gpdb,yuanzhao/gpdb,greenplum-db/gpdb,50wu/gpdb,jmcatamney/gpdb,edespino/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,50wu/gpdb,50wu/gpdb,Chibin/gpdb,50wu/gpdb,lisakowen/gpdb,edespino/gpdb,yuanzhao/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,adam8157/gpdb,lisakowen/gpdb,yuanzhao/gpdb,adam8157/gpdb,adam8157/gpdb,adam8157/gpdb,jmcatamney/gpdb,edespino/gpdb,Chibin/gpdb,greenplum-db/gpdb,edespino/gpdb,xinzweb/gpdb,yuanzhao/gpdb,adam8157/gpdb,xinzweb/gpdb