warning: a Mix listener expected by Phoenix.CodeReloader is missing during debug

Phoenix 1.8.0-rc.3
```
warning: a Mix listener expected by Phoenix.CodeReloader is missing.

Please add the listener to your mix.exs configuration, like so:

def project do
[
...,
listeners: [Phoenix.CodeReloader]
]
end

(phoenix 1.8.0-rc.3) lib/phoenix/code_reloader/server.ex:76: anonymous fn/5 in Phoenix.CodeReloader.Server.handle_call/3
(mix 1.18.3) lib/mix/sync/lock.ex:112: Mix.Sync.Lock.with_lock/3
(phoenix 1.8.0-rc.3) lib/phoenix/code_reloader/server.ex:70: Phoenix.CodeReloader.Server.handle_call/3
(stdlib 6.2.2) gen_server.erl:2381: :gen_server.try_handle_call/4
(stdlib 6.2.2) gen_server.erl:2410: :gen_server.handle_msg/6
(stdlib 6.2.2) proc_lib.erl:329: :proc_lib.init_p_do_apply/3
```
Then debug stops when a file is modified and saved.