Remove Potential SQL injection vulnerabilities from Redshift class

[HandcartCactus]

parsons/parsons/databases/redshift/redshift.py

Lines 1154 to 1155 in 3bedb1a

	source_ns.nspname = '{schema}'
	and source_table.relname = '{table}'

[HandcartCactus]

While it's unlikely drop_dependencies_for_cols would be used in a way that's exploitable, it should prooooobably just use proper parameterization, because it can. I'm poking around in this class for #364 so I'd gladly fix this one once I'm done with the other ticket.