Make sure nodejs plugin uses specified version of npm

currently the nodejs plugin uses bundled version of npm on linux/osx and the specified version of npm on windows. with the update of npm to 2.x version, it is not possible to download a package of npm with the version starting from 2.x (see https://github.com/npm/npm#windows-install-or-upgrade)

therefore the plugin should be enhanced to always use the specified version of npm regardless of the environment