npm: upgrade to 1.3.20

The 1.3.19 release had a critical bug: any packages published with it
could not be installed, because the shasum would be incorrect.

Thankfully, 1.3.19 was published using 1.3.19, so could not be installed
by any users!  However, if it goes out as part of a Node.js release,
then obviously that would be a problem.