Fix require_map_reduce mim import

We don't import mim at the top level file anymore, so there's no way to
check against mim.Connection. And we can't blindly check for conn.conn since
other drivers like SQLAlchemy don't have a conn attributes. That's not part
of the Connection interface.

Change-Id: I25ea31d10bb7a28296676fbec4d2ad015d082cf1
Signed-off-by: Julien Danjou <julien@danjou.info>
Merge "Fix require_map_reduce mim import"