Docs: Change resp_body to sent_body

`f849b33f` changed `Dynamo.HTTP.Test` to return the body as `sent_body` instead of `resp_body`. Changes docs to reflect that change.
Merge pull request #153 from michihuber/patch-1

Docs: change resp_body to sent_body
Update to Elixir v0.11.1-dev
Changed setelem to set_elem
Merge pull request #154 from marcioj/patch-1

Changed setelem to set_elem
Make use of build_per_environment