phoenix 1.4 compatability

It looks like it is not an easy upgrade to 1.4 within an umbrella app.

```
Failed to use "phoenix_ecto" (version 4.0.0) because

`phoenix_ecto` 
  deps/ex_admin/mix.exs requires ~> 3.2

Failed to use "cowboy" (versions 2.0.0 to 2.6.1) because
`cowboy` -> `plug_cowboy`
  deps/ex_admin/mix.exs requires ~> 1.0
```