I want to run tests of my Phoenix app on Travis-CI.

Log excerpt:

```
$ MIX_ENV=test mix do deps.get, compile, test

Could not find hex, which is needed to build dependency :phoenix

Shall I install hex? [Yn] 

```

When it comes to fetching and installing dependencies, it asks if it should install `hex`. I was wondering if I can pass a `--yes` option to mix so that it doesn't ask but just installs?