MDL-55048 travis: check npm/grunt on multiple npm versions
MDL-55048 travis: check npm/grunt on multiple npm versions

Check npm install and grunt execution on the higest and lowest node
versions. Included some minor tweaks along the way:

* Move the grunt execution into scripts part of the configuration (that way
  the grunt output is not collapsed and is easily visible in build output).

* Restructure the install section to be task based (thanks to Eloy for
  the suggestion) and as such, don't run composer install on the grunt
  task (speed up the build slightly).

* Switch around the grunt job to a matrix include so that every
  variation of grunt job doesn't need to be excluded.
  ( https://docs.travis-ci.com/user/customizing-the-build/#Explicity-Including-Jobs )

dilawar/moodle