Clearer mentions of util.js

Created by: sodevious

I realize a simialr issue has been made and closed: https://github.com/twbs/bootstrap/issues/17193

However, the collapse.js plugin page should mention that util.js is a dependency.

Additionally, the reference to transitions.js on the Introduction page should be changed to the appropriate util.js reference: http://v4-alpha.getbootstrap.com/getting-started/introduction/