Specify erlang 19 as dependency in the docs for using docker socket access

[elpddev]

When trying to use docker socket access, Erlang v19 and above is required. The error messages getting from httpoison are not very helpful, so it's a bit hard for users to understand how to solve the problem.

https://github.com/edgurgel/httpoison/releases/tag/v0.10.0

HTTPoison 0.10.0
Add support for http over unix sockets - requires hackney >= 1.6.3, erlang >= 19. #185 .