fix drop materialized view, missing cascade

[lvitti]

resolves dbt-labs#642

Problem

When you refresh the materialized view but there are other views that depend on it, an error occurs.

Solution

Add the cascade to the drop like already do for Table and view

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