preinstall: update peerDeps before global install

Using node as a portable scripting language, install adequate versions
of our peerDependencies globally when we are installed globally
ourselves.

@sam-github an alternative to `slc update`?
 - It should be portable because it is node, not bash
 - It can install preferred versions since it runs after package.json has been expanded
 - It _should_ "just work" with `npm -g update strong-cli`