use default http client

`&http.Client{}` actually generates the `http.DefaultClient`, which can be
used concurrently already.

In order to re-use existing idle connections to aws and generate less garbage,
just use http.DefaultClient.

The proper connection management and idle pooling is already provided by the
default transport.

@tobstarr please take a look. If you explicitly want to avoid the pooling for certain reasons, please tell me.

net/http is really awesome by default :-)