Respects ResultProxy.supports_sane_rowcount()

Respects ResultProxy.supports_sane_rowcount()

The check for matched rowcount when the alembic_version table is updated or
deleted from is now conditional based on whether or not the dialect
supports the concept of "rowcount" for UPDATE or DELETE rows matched.  Some
third party dialects do not support this concept.  Pull request courtesy Ke
Zhu.

Closes: #667
Pull-request: https://github.com/sqlalchemy/alembic/pull/667
Pull-request-sha: a0d45ed0f5de582314faae2210eeec881670488e

Change-Id: I09c9b540d8e21a94728085270eb20ba2273cbdb1