update_status: first positional argument should be 'status'

update_status: first positional argument should be 'status'

Passing the tweet text as the lone positional argument to update_status:

     api.update_status(u'Hi mum!')

is suggested in much documentation and in several tests, but f99b1da
introduced media_ids as the first argument, breaking this API.

This change will break anyone who uses tweepy >= 3.2 and assumes they
can pass media_ids as the first positional; but I expect that to be a
much smaller set than people who have followed the example in the docs,
who find their application mysteriously fails to post tweets after
upgrading tweepy.

Fixes #554.