Posting this in case anyone else runs into the same issue.
If after installing Phoenix and creating a new app (and installing dependencies) you get this error:
Fetch and install dependencies? [Yn] Y
- running mix deps.get
- running mix deps.compile
- running cd assets && npm install && node node_modules/brunch/bin/brunch build
We are almost there! The following steps are missing:
$ cd gallows
$ cd assets && npm install && node node_modules/brunch/bin/brunch build
Start your Phoenix app with:
$ mix phx.server
You can also run your app inside IEx (Interactive Elixir) as:
$ iex -S mix phx.server
It means you haven’t installed Node or are running an old version of it.
Solution: install Node 
On Mac, you can simply:
brew install node
or
brew upgrade node
Then make sure you:
$ cd gallows
$ cd assets && npm install && node node_modules/brunch/bin/brunch build
Check everything’s running by going back into your app’s parent directory (cd ..) and then:
mix phx.server
