[cmake] update requirement and guides

[jgvictores]

• https://github.com/robotology/QA/issues/364
• https://askubuntu.com/questions/952429/is-there-a-good-ppa-for-cmake-backports
• https://github.com/roboticslab-uc3m/questions-and-answers/issues/84#issuecomment-559400444

[PeterBowman]

Preferred way: use Kitware's PPA (answer). We are not going to support Ubuntu Trusty (14.04) anymore (concerning this answer).