We should install elm to successfully build atc
We should install elm to successfully build atc

The build fails due to missing an elm binary without `npm install`ing it:

```
$ make -B
cd elm && elm make --warn --output ../public/elm.js --yes src/Main.elm
/bin/sh: elm: command not found
make: *** [public/elm.js] Error 127
```
concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse