function getLatestNPMVersion () {
  return new Promise((resolve, reject) => {
    exec('npm show npm version', (err, stdout) => {
      if (err) {
        let error = 'We could not show latest available versions. Try running this script again '
        error += 'with the version you want to install (npm-windows-upgrade --npm-version 3.0.0)'
        return reject(error)
      }

      let latest = stdout.replace(/(\r\n|\n|\r)/gm, '')

      resolve(latest.trim())
    })
  })
}
Fetches the published versions of npm from the npm registry
@return {Promise.<version>} - Array of the available versions