(maint) change json dep from jazz to poison

Prior to this commit, we were using Jazz as the JSON parser for the
emoji data; however, Jazz appears to abandoned and Poison has been under
active development and is the de facto JSON parsing library for many of
the high profile Elixir frameworks, like Phoenix.

lib/exmoji.ex

@@ -20,7 +20,7 @@ defmodule Exmoji do
   @external_resource vendor_data_file
 
   rawfile = File.read! vendor_data_file
-  rawdata = Jazz.Decode.it! rawfile, keys: :atoms
+  rawdata = Poison.decode! rawfile, keys: :atoms
   emoji_chars = for char <- rawdata do
     %EmojiChar{
       name:         char.name,

mix.exs

@@ -45,7 +45,7 @@ defmodule Exmoji.Mixfile do
   # Type `mix help deps` for more examples and options
   defp deps do
     [
-      {:jazz,         "~> 0.2.1"},
+      {:poison,       "~> 1.5"},
       {:excoveralls,  "~> 0.3",                   only: :dev},
       {:benchfella,   github: "alco/benchfella",  only: :dev},
       {:earmark,      "~> 0.1",                   only: :dev},