MDL-55048 grunt/npm: Update dependencies

* We bump our minimum node version up to >=4. Although this isn't yet
  necessarily *required* it was spurred on because grunt dropped support
  for very early node versions and so it's likely some devs will have to
  upgrade their npm.

  We do it now which gives us more flexibility for tools going forward
  (especially for things like new theme/testing). And node has matured
  the ecosystem (especially supporting properly semver) so should result
  in less edge cases.

* We make the minimum node version explicit in package.json. Though it
  doesn't help much for older versions of node because npm doesn't
  have the functionality to warn about it.. ;-)

* Left async dependency as it was, because it's only on an RC: 2.0.0-rc.6

lib/upgrade.txt

@@ -3,6 +3,7 @@ information provided here is intended especially for developers.
 
 === 3.2 ===
 
+* Node.js versions >=4 are now required to run grunt.
 * New option 'blanktarget' added to format_text. This option adds target="_blank" to links
 * A new webservice structure `external_files` has been created which provides a standardised view of files in Moodle and
   should be used for all file return descriptions.