Revert "Merge pull request #49 from mczepiel/child-process"

This reverts commit b112197c5b7f5b71afb7008dbe3e65c396c5fee8, reversing
changes made to 1864004b0bb98f31bf13aa25face0b03f7b7ace1.

This was blindly using whichever `npm` binary happened to be
found through the environment, not necessarily the one found
within minit itself.
Update npm dependency to the latest

If only to minimize the concern that things have been addressed when
trying to debug issues.
Manually set the npm prefix before running install

I had tried all the other configuration setting approaches yesterday to
no avail, and I thought I'd checked this one. But either way, this is
a lot easier than forking a new npm process, especially when we want to definitely rely on the npm packaged as part of minit.

It is troubling that any other application that has multiple libraries
working with npm might need to step in to coordinate them from trashing
each other's configurations.