slugbuilder: Use npm-next branch of nodejs buildpack

This installs a newer, less broken version of npm.

https://github.com/heroku/heroku-buildpack-nodejs/compare/v60...fae41898 

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>
Merge pull request #583 from flynn/npm-next

slugbuilder: Use npm-next branch of nodejs buildpack