Update install.rst (#614)
Update install.rst (#614)

Added command for installation all the prerequisites if user is on Ubuntu 17.04.
mathjazz/pontoon,mozilla/pontoon,jotes/pontoon,jotes/pontoon,mastizada/pontoon,mastizada/pontoon,jotes/pontoon,mathjazz/pontoon,mozilla/pontoon,mathjazz/pontoon,jotes/pontoon,mastizada/pontoon,mathjazz/pontoon,mozilla/pontoon,mozilla/pontoon,mathjazz/pontoon,mozilla/pontoon,mastizada/pontoon