Clearer mentions of util.js

[sodevious]

I realize a simialr issue has been made and closed: #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/

[pvdlg]

Changed the reference to transition.js to util.js in #21743 (although the PR covert primarily something else)

[bastienmoulia]

https://github.com/twbs/bootstrap/blob/v4-dev/docs/getting-started/javascript.md#util

Seems good to me.