Just fix the npm bug, on 0.10.29

Even though 0.10.29 has a security hole, 0.10.30 is worse: it often
takes 100% CPU if you pass a float to setTimeout!
Only rebuild npm modules on major/minor Node bumps

This means that we won't have to bump the version number of every
npm-using package every time we do a patch update of Node. (Though we
will have to do it right now, once.)  (That's because the .node_version
file is part of the package's contents... which is semantically
correct.)