Better stack traces

[paulanthonywilson]

Adds the location: :keep option to the quote in the __using__/1 macro. These gives better stack traces if and when unexpected errors are raised somewhere in the code. Otherwise the stack tops out at use HTTPoison.Base in HTTPoison or whever the base is being used.

The PR also includes:

• updating ssl_verify_fun as otherwise I can't get things to compile (at least with Elixir 1.5 / Erlang 26 on OS X and Ubuntu)
• Fixed a couple of tests that I think are failing for me because of Map key ordering changing with Elixir 1.5 (or is it Erlang 26?)

Oh, and it looks like mix format has gone and changed a bunch of charlist strings to use the ~c sigil. ¯\(ツ)/¯ . I can revert that if you like.

"test ssl config tests https scheme" in HTTPoisonTest is still failing in Elixir 1.5 but I don't know why. Something to do with httparrot and ssl.

[edgurgel]

Thank you! This is great