rub -b does not minifiy plugins' libraries

[moloko]

• Download a standard adapt OS setup via adapt create course rub-cli-test
• run grunt build
• Note how files like the following are all minified
  • mediaelement-and-player.js
  • rangeslider.js
  • select2.js
• now run rub -b
• note how those files are no longer minified

On an OOTB Adapt build, minifying these files reduces the size of the libraries folder by over 250KB so I think this is worth sorting out