Makes greenplum db-driver ensure psql is Greenplum's
Makes greenplum db-driver ensure psql is Greenplum's

Fixes #553
Fixes #494

HazyResearch/deepdive,HazyResearch/deepdive,HazyResearch/deepdive,shahin/deepdive,sky-xu/deepdive,sky-xu/deepdive,HazyResearch/deepdive,shahin/deepdive