Phoenix 1.4 breaks ESpec

I have channel tests using `Phoenix.ChannelTest` which is broken in ESpec and Phoenix 1.4.0.

It crashes with:

```
** (ArgumentError) could not fetch application environment :refute_receive_timeout for application :ex_unit because the application was not loaded/started. If your application depends on :ex_unit at runtime, make sure to load/start it or list it under :extra_applications in your mix.exs file
```

It happens so, because previosly, they received timeout for `refute_broadcast` function from argument: https://hexdocs.pm/phoenix/1.3.4/Phoenix.ChannelTest.html#refute_broadcast/3,
but in 1.4.0, they try to fetch `:ex_unit` application, which fails.

Also, `:ex_unit` application can't be start because of:

```
** (MatchError) no match of right hand side value: {:error, {:ex_unit, {{:shutdown, {:failed_to_start_child, ExUnit.CaptureServer, {:already_started, #PID<0.496.0>}}}, {ExUnit, :start, [:normal, []]}}}}
```

since espec starts ExUnit.CaptureServer on its side