When playing with a Rainbow Pool syncing task, I noticed that error
handling wasn't working correctly for requests using `async-http`: if
the block raised an error, the `Sync` block would never return.
My suspicion is that this is because we were never reading or releasing
the request body.
In this change, I upgrade all the relevant gems for good measure, and
switch to using the response object yielded by the _block_, so we can
know it's being resource-managed correctly. Now, failures raise errors
as expected!
(I tested all these relevant service calls, too!)
