Update readme npm instructions

This is a quick fix that backports part of the readme updates we're making in v3 to address the current out of date npm installation process. With v2.3 we overhauled the makefile to only install dependencies locally, and thus we don't need to install them globally via `-g` anymore. Problem is we forgot to update the makefile before release.

From now on, just run `npm install`. If you run into problems, be sure to uninstall all the old versions and try again.

/cc @cvrebert