Add appsignal to deps and application in mix.exs

After adding the appsignal package to mix.exs, as per
https://github.com/appsignal/appsignal-elixir#readme, dependencies won't
compile because of lib/appsignal/priv/libappsignal.dylib being missing:

    ~/opensource/appsignal_phoenix_example $ mix deps.install
    ~/opensource/appsignal_phoenix_example $ mix deps.get
    Running dependency resolution
    Dependency resolution completed
      appsignal: 0.0.3
    * Getting appsignal (Hex package)
      Checking package (https://repo.hex.pm/tarballs/appsignal-0.0.3.tar)
      Using locally cached package
    ~/opensource/appsignal_phoenix_example $ mix deps.compile
    ==> connection
    Compiling 1 file (.ex)
    Generated connection app
    ==> fs (compile)
    Compiled src/sys/inotifywait_win32.erl
    Compiled src/sys/fsevents.erl
    Compiled src/sys/inotifywait.erl
    Compiled src/fs_event_bridge.erl
    Compiled src/fs_sup.erl
    Compiled src/fs_app.erl
    Compiled src/fs_server.erl
    Compiled src/fs.erl
    Compiling c_src/mac/cli.c
    Compiling c_src/mac/compat.c
    Compiling c_src/mac/main.c
    ==> gettext
    Compiling 1 file (.yrl)
    Compiling 1 file (.erl)
    Compiling 19 files (.ex)
    Generated gettext app
    ==> ranch (compile)
    Compiled src/ranch_transport.erl
    Compiled src/ranch_sup.erl
    Compiled src/ranch_server.erl
    Compiled src/ranch_protocol.erl
    Compiled src/ranch_listener_sup.erl
    Compiled src/ranch_conns_sup.erl
    Compiled src/ranch_tcp.erl
    Compiled src/ranch_ssl.erl
    Compiled src/ranch_app.erl
    Compiled src/ranch_acceptor.erl
    Compiled src/ranch_acceptors_sup.erl
    Compiled src/ranch.erl
    ==> poolboy (compile)
    Compiled src/poolboy_sup.erl
    Compiled src/poolboy_worker.erl
    Compiled src/poolboy.erl
    ==> decimal
    Compiling 1 file (.ex)
    Generated decimal app
    ==> poison
    Compiling 4 files (.ex)
    Generated poison app

    14:53:09.366 [info]  Downloading agent release from https://appsignal-agent-releases.global.ssl.fastly.net/dacf2fb/appsignal-x86_64-darwin-all-static.tar.gz
    clang: error: no such file or directory: '/Users/jeff/opensource/appsignal_phoenix_example/_build/dev/lib/appsignal/priv/libappsignal.dylib'
    make: *** [all] Error 1
    ==> appsignal
    could not compile dependency :appsignal, "mix compile" failed. You can recompile this dependency with "mix deps.compile appsignal", update it with "mix deps.update appsignal" or clean it with "mix deps.clean appsignal"
    ==> appsignal_phoenix_example
    ** (Mix) Could not run `make`. Please check if `make` and either `clang` or `gcc` are installed

mix.exs

@@ -19,7 +19,7 @@ defmodule AppsignalPhoenixExample.Mixfile do
   def application do
     [mod: {AppsignalPhoenixExample, []},
      applications: [:phoenix, :phoenix_pubsub, :phoenix_html, :cowboy, :logger, :gettext,
-                    :phoenix_ecto, :postgrex]]
+                    :phoenix_ecto, :postgrex, :appsignal]]
   end
 
   # Specifies which paths to compile per environment.
@@ -38,6 +38,7 @@ defmodule AppsignalPhoenixExample.Mixfile do
      {:phoenix_live_reload, "~> 1.0", only: :dev},
      {:gettext, "~> 0.11"},
      {:cowboy, "~> 1.0"},
+     {:appsignal, "~> 0.0.0"}
    ]
   end