Add httpoison as explicit dependency
Add httpoison as explicit dependency

Otherwise, it won't get installed by `mix deps.get` with
the `--only` switch, which is used by the heroku build pack.

https://github.com/HashNuke/heroku-buildpack-elixir/blob/aea1c4953de0354a6c09b9b9aa5f249575770a61/lib/app_funcs.sh#L79

hashrocket/tilex,hashrocket/tilex,hashrocket/tilex