Add cascade to drop mv

[mikealfare]

resolves #642

Problem

We drop materialized views with a standard DROP instead of a DROP ... CASCADE. When a dbt project has materialized views that reference other materialized views, the materialized view cannot be dropped during a full refresh. This raises a database error.

Solution

Update the DROP statement to a DROP ... CASCADE statement.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX