address phoenix test warnings

[jsegal205]

Running mix test threw the following warnings:

$ mix test
Generated bamboo_phoenix app
warning: Phoenix now requires you to explicitly list which engine to use
for Phoenix JSON encoding. We recommend everyone to upgrade to
Jason by setting in your config/config.exs:

    config :phoenix, :json_library, Jason

And then adding {:jason, "~> 1.0"} as a dependency.

If instead you would rather continue using Poison, then add to
your config/config.exs:

    config :phoenix, :json_library, Poison

  (phoenix 1.5.7) lib/phoenix.ex:38: Phoenix.start/2
  (kernel 9.0) application_master.erl:293: :application_master.start_it_old/4

....warning: Rendering the child template from layouts is deprecated. Instead of:

    <%= render(@view_module, @view_template, assigns) %>

You should do:

    <%= @inner_content %>

  (phoenix 1.5.7) lib/phoenix/view.ex:283: Phoenix.View.render/3
  test/support/templates/phoenix_layout/app.html.eex:2: Bamboo.PhoenixTest.PhoenixLayoutView."app.html"/1
  (phoenix 1.5.7) lib/phoenix/view.ex:472: Phoenix.View.render_to_iodata/3
  (phoenix 1.5.7) lib/phoenix/view.ex:479: Phoenix.View.render_to_string/3
  (bamboo_phoenix 1.0.0) lib/bamboo_phoenix.ex:282: Bamboo.Phoenix.render_html_and_text_emails/1
  test/bamboo_phoenix_test.exs:63: Bamboo.PhoenixTest."test render/2 allows setting a custom layout"/1
  (ex_unit 1.16.3) lib/ex_unit/runner.ex:472: ExUnit.Runner.exec_test/2
  (stdlib 5.0) timer.erl:270: :timer.tc/2
  (ex_unit 1.16.3) lib/ex_unit/runner.ex:394: anonymous fn/6 in ExUnit.Runner.spawn_test_monitor/4

warning: Rendering the child template from layouts is deprecated. Instead of:

    <%= render(@view_module, @view_template, assigns) %>

You should do:

    <%= @inner_content %>

  (phoenix 1.5.7) lib/phoenix/view.ex:283: Phoenix.View.render/3
  test/support/templates/phoenix_layout/app.text.eex:2: Bamboo.PhoenixTest.PhoenixLayoutView."app.text"/1
  (phoenix 1.5.7) lib/phoenix/view.ex:472: Phoenix.View.render_to_iodata/3
  (phoenix 1.5.7) lib/phoenix/view.ex:479: Phoenix.View.render_to_string/3
  (bamboo_phoenix 1.0.0) lib/bamboo_phoenix.ex:283: Bamboo.Phoenix.render_html_and_text_emails/1
  test/bamboo_phoenix_test.exs:63: Bamboo.PhoenixTest."test render/2 allows setting a custom layout"/1
  (ex_unit 1.16.3) lib/ex_unit/runner.ex:472: ExUnit.Runner.exec_test/2
  (stdlib 5.0) timer.erl:270: :timer.tc/2
  (ex_unit 1.16.3) lib/ex_unit/runner.ex:394: anonymous fn/6 in ExUnit.Runner.spawn_test_monitor/4

....
Finished in 0.1 seconds (0.00s async, 0.1s sync)
8 tests, 0 failures

reference the phoenix 1.5 changes for the layout updates - https://gist.github.com/chrismccord/e53e79ef8b34adf5d8122a47db44d22f#update-your-layouts

This addresses those warnings -

• updates test files
• updates docs
• explicitly adds Jason dep and list in the config file.