Fix location of npm install.sh.

The new location is `www.npmjs.org/install.sh`. curl is not configured
to follow a redirection.
Teach curl to follow a redirection

While the previous commit fix the location of npm install.sh, we ensure
that the problem won't bite us again in the future by asking curl to
follow the redirection if any.
Merge pull request #52 from vincentbernat/fix/npmjs-redirect

fixed npm.js redirect