Do not assume "postgres" user exists

Revert "CDB-3079 enforces postgres user in preparation script"
Revert "CDB-3079 enforces postgres user in preparation script"
[TRAVIS] set "postgres" user for running tests (using PGUSER)

Closes #152
Merge branch 'no-libpq-param-enforcements'

Conflicts:
	NEWS.md
	test/prepare_db.sh