Support `postgresql_concurrently` on index dropping.

Support `postgresql_concurrently` on index dropping.

Also adds version detection so that DROP INDEX CONCURRENTLY
only renders if the Postgresql version is 9.2 or higher;
for CREATE INDEX CONCURRENTLY, version detection is added
to allow the phrase to omit if the Postgresql version is
less than 8.2.

Co-authored-by: Mike Bayer <mike_mp@zzzcomputing.com>
Change-Id: I597287e0ebbbe256c957a3579b58ace6848ab4f4
Pull-request: https://bitbucket.org/zzzeek/sqlalchemy/pull-requests/84