Use nodejs buildpack directly (instead of via the emberjs buildpack)

Since #2657 we build the frontend twice during a deploy. It seems that
the nodejs buildpack (pulled in by the emberjs buildpack) is running
our build script so lets just depend on that directly because it is
actively supported by Heroku.

Author: jtgeibel

.buildpacks

@@ -1,4 +1,4 @@
 https://github.com/emk/heroku-buildpack-rust#578d630
-https://jtgeibel.com/emberjs.tgz
+https://github.com/heroku/heroku-buildpack-nodejs#v176
 https://github.com/heroku/heroku-buildpack-nginx.git#fbc49cd
 https://github.com/sgrif/heroku-buildpack-diesel#f605edd