fix warning because jason app is not declared as dependency

[asmodehn]

With recent elixir versions (1.11 and 1.12), I get :

==> inch_ex
Compiling 27 files (.ex)
warning: Jason.encode!/2 defined in application :jason is used by the current application but the current application does not depend on :jason. To fix this, you must do one of:

  1. If :jason is part of Erlang/Elixir, you must include it under :extra_applications inside "def application" in your mix.exs

  2. If :jason is a dependency, make sure it is listed under "def deps" in your mix.exs

  3. In case you don't want to add a requirement to :jason, you may optionally skip this warning by adding [xref: [exclude: [Jason]]] to your "def project" in mix.exs

  lib/inch_ex/json.ex:5: InchEx.JSON.encode!/1

The fix consists in letting mix expand the dependent applications from the list of dependencies instead of overriding them with the :applications key in the project application definition.

Ref : https://hexdocs.pm/mix/Mix.Tasks.Compile.App.html