Django database backends can specify the SQL they want to use for

dropping indexes. We now use this when running on Django 1.7+ to drop

indexes, instead of hard-coding a SQL statement. This primarily impacts

Postgres on Django 2.0+, which adds an IF EXISTS to the statement

(likely to deal with all the variants in index names that Django has

used over the years for Postgres).

Unit tests pass on Django 1.6 through 1.11 on all supported database

types, and on 2.1 with other pending changes.