Run CI with unstable Node.js as an allowed failure

Building against the latest unstable release of Node.js will help the
project maintain awareness of any regressions or breaking changes in the
platform. Specifying that build as an allowed failure guarantees that
unforeseen errors do not block ongoing work.

Closes gh-1074.
Reverting fix for isaacs/npm#3958. Closes gh-1046, gh-1077.

* This doesn't work when "npm install" is run as root.
* When devving grunt, the user should follow "npm install" with "npm uninstall grunt" to work around the isaacs/npm#3958 issue. This will be reflected in the docs.