Use response.text for deserialization

Under Python 2, `json.loads` could accept either `response.content` or
`response.text` as both held a `str` object -- even though `content`
held binary data while `text` held the decoded content.

Under Python 3, this doesn't work as `response.content` is a bytes
object while `response.text` is unicode.

So, with this commit, use `response.text` in `json.loads` so it's
always passed the decoded content.

Refs: #5