You aren’t using Phoenix 1.4 but an earlier version.
In 1.4 phoenix.css includes milligram 1.3.0 and later defines the phx-hero class.
In 1.3.2 phoenix.css includes Bootstrap 3.3.5 which defines the jumbotron class.
https://github.com/phoenixframework/phoenix/blob/v1.3/installer/templates/phx_assets/phoenix.css#L9
I describe how to convert a 1.3.2 project from Brunch to webpack here - which also uses the new assets.
