Fix upper case bucket names regression (from #3).

Evidently, boto.storage_uri called without extra connection arguments
constructed a connection object compatible with uppercase bucket names
(which I discovered today is only allowed in us-east-1; one gets an
error attempting to create a bucket with an uppercase name in all other
regions).  Issue #3 added a 'host' connection argument that appears to
have changed the configuration.  Explicitly request the appropriate
calling format.

Signed-off-by: Daniel Farina <daniel@heroku.com>