All of a sudden my app has started to crash:

    $ mix phx.server
    [................]

    Generated my_app app
    [info] Application my_app exited: MyApp.Application.start(:normal, []) returned an error: shutdown: failed to start child: MyAppWeb.Endpoint
        ** (EXIT) shutdown: failed to start child: Phoenix.Endpoint.Handler
            ** (EXIT) an exception was raised:
                ** (UndefinedFunctionError) function Phoenix.Endpoint.Cowboy2Handler.child_spec/3 is undefined (module Phoenix.Endpoint.Cowboy2Handler is not available)
                    Phoenix.Endpoint.Cowboy2Handler.child_spec(:http, MyAppWeb.Endpoint, [port: 4000, otp_app: :my_app])
                    (phoenix) lib/phoenix/endpoint/handler.ex:33: anonymous fn/5 in Phoenix.Endpoint.Handler.init/1
                    (elixir) lib/enum.ex:1925: Enum."-reduce/3-lists^foldl/2-0-"/3
                    (phoenix) lib/phoenix/endpoint/handler.ex:31: Phoenix.Endpoint.Handler.init/1
                    (stdlib) supervisor.erl:295: :supervisor.init/1
                    (stdlib) gen_server.erl:374: :gen_server.init_it/2
                    (stdlib) gen_server.erl:342: :gen_server.init_it/6
                    (stdlib) proc_lib.erl:249: :proc_lib.init_p_do_apply/3
    ** (Mix) Could not start application my_app: MyApp.Application.start(:normal, []) returned an error: shutdown: failed to start child: MyAppWeb.Endpoint
        ** (EXIT) shutdown: failed to start child: Phoenix.Endpoint.Handler
            ** (EXIT) an exception was raised:
                ** (UndefinedFunctionError) function Phoenix.Endpoint.Cowboy2Handler.child_spec/3 is undefined (module Phoenix.Endpoint.Cowboy2Handler is not available)
                    Phoenix.Endpoint.Cowboy2Handler.child_spec(:http, MyAppWeb.Endpoint, [port: 4000, otp_app: :my_app])
                    (phoenix) lib/phoenix/endpoint/handler.ex:33: anonymous fn/5 in Phoenix.Endpoint.Handler.init/1
                    (elixir) lib/enum.ex:1925: Enum."-reduce/3-lists^foldl/2-0-"/3
                    (phoenix) lib/phoenix/endpoint/handler.ex:31: Phoenix.Endpoint.Handler.init/1
                    (stdlib) supervisor.erl:295: :supervisor.init/1
                    (stdlib) gen_server.erl:374: :gen_server.init_it/2
                    (stdlib) gen_server.erl:342: :gen_server.init_it/6
                    (stdlib) proc_lib.erl:249: :proc_lib.init_p_do_apply/3

I’m using and have been using for a while the latest versions of everything: Cowboy - 2.5, Phoenix - 1.3. All has been working fine for over a half a year