function getInstalledNPMVersion () {
  return new Promise((resolve, reject) => {
    let nodeVersion

    exec('npm -v', (err, stdout) => {
      if (err) {
        reject(new Error('Could not determine npm version.'))
      } else {
        nodeVersion = stdout.replace(/\n/, '')
        resolve(nodeVersion)
      }
    })
  })
}
Gets the currently installed version of npm (npm -v)
@return {Promise.<string>} - Installed version of npm