Replace poison dependency w/ jason

The dependency took a long time to compile, and was causing problems for
a user who was attempting to build the project.

Since it wasn't a strictly necessary dependency, and `jason` was already
included in the project, all instances of `poison` have been replaced
with `jason`.

The only additional code that this introduced was converting from
generic maps returned by `Jason.decode` into Service structs.

test/farside_test.exs

@@ -45,7 +45,7 @@ defmodule FarsideTest do
 
   test "/:service" do
     {:ok, file} = File.read(@services_json)
-    {:ok, service_list} = Poison.decode(file, as: [%{}])
+    {:ok, service_list} = Jason.decode(file)
 
     service_names =
       Enum.map(