193 Upgrade Poison to version 4.0.1

[soriyath]

I've tried solving this issue: https://github.com/BlakeWilliams/Elixir-Slack/issues/193

[soriyath]

The failed check is because Poison v4.0.1 requires Elixir 1.6.0 at least, and the failed check runs on Elixir 1.4.x

Elixir 1.4.0
install.rebar
1.18s$ mix local.rebar --force
install.hex
0.88s$ mix local.hex --force
install.deps
1.67s$ mix deps.get
14.87s$ mix test
===> Compiling parse_trans
===> Compiling websocket_client
===> Compiling mimerl
===> Compiling metrics
==> bunt
Compiling 2 files (.ex)
Generated bunt app
===> Compiling unicode_util_compat
===> Compiling idna
===> Compiling ranch
==> poison
warning: the dependency :poison requires Elixir "~> 1.6" but you are running on v1.4.0
Compiling 4 files (.ex)
== Compilation error on file lib/poison/parser.ex ==
** (CompileError) lib/poison/parser.ex:276: undefined function defguardp/1
could not compile dependency :poison, "mix compile" failed. You can recompile this dependency with "mix deps.compile poison", update it with "mix deps.update poison" or clean it with "mix deps.clean poison"
The command "mix test" exited with 1.
Done. Your build exited with 1.

[BlakeWilliams]

Thanks for tackling this! 👾